Bake file reference
The Bake file is a file for defining workflows that you run using docker buildx bake
.
File format
You can define your Bake file in the following file formats:
- HashiCorp Configuration Language (HCL)
- JSON
- YAML (Compose file)
By default, Bake uses the following lookup order to find the configuration file:
compose.yaml
compose.yml
docker-compose.yml
docker-compose.yaml
docker-bake.json
docker-bake.override.json
docker-bake.hcl
docker-bake.override.hcl
You can specify the file location explicitly using the --file
flag:
$ docker buildx bake --file ../docker/bake.hcl --print
If you don't specify a file explicitly, Bake searches for the file in the
current working directory. If more than one Bake file is found, all files are
merged into a single definition. Files are merged according to the lookup
order. That means that if your project contains both a compose.yaml
file and
a docker-bake.hcl
file, Bake loads the compose.yaml
file first, and then
the docker-bake.hcl
file.
If merged files contain duplicate attribute definitions, those definitions are either merged or overridden by the last occurrence, depending on the attribute. The following attributes are overridden by the last occurrence:
target.cache-to
target.dockerfile-inline
target.dockerfile
target.outputs
target.platforms
target.pull
target.tags
target.target
For example, if compose.yaml
and docker-bake.hcl
both define the tags
attribute, the docker-bake.hcl
is used.
$ cat compose.yaml
services:
webapp:
build:
context: .
tags:
- bar
$ cat docker-bake.hcl
target "webapp" {
tags = ["foo"]
}
$ docker buildx bake --print webapp
{
"group": {
"default": {
"targets": [
"webapp"
]
}
},
"target": {
"webapp": {
"context": ".",
"dockerfile": "Dockerfile",
"tags": [
"foo"
]
}
}
}
All other attributes are merged. For example, if compose.yaml
and
docker-bake.hcl
both define unique entries for the labels
attribute, all
entries are included. Duplicate entries for the same label are overridden.
$ cat compose.yaml
services:
webapp:
build:
context: .
labels:
com.example.foo: "foo"
com.example.name: "Alice"
$ cat docker-bake.hcl
target "webapp" {
labels = {
"com.example.bar" = "bar"
"com.example.name" = "Bob"
}
}
$ docker buildx bake --print webapp
{
"group": {
"default": {
"targets": [
"webapp"
]
}
},
"target": {
"webapp": {
"context": ".",
"dockerfile": "Dockerfile",
"labels": {
"com.example.foo": "foo",
"com.example.bar": "bar",
"com.example.name": "Bob"
}
}
}
}
Syntax
The Bake file supports the following property types:
target
: build targetsgroup
: collections of build targetsvariable
: build arguments and variablesfunction
: custom Bake functions
You define properties as hierarchical blocks in the Bake file. You can assign one or more attributes to a property.
The following snippet shows a JSON representation of a simple Bake file. This Bake file defines three properties: a variable, a group, and a target.
{
"variable": {
"TAG": {
"default": "latest"
}
},
"group": {
"default": {
"targets": ["webapp"]
}
},
"target": {
"webapp": {
"dockerfile": "Dockerfile",
"tags": ["docker.io/username/webapp:${TAG}"]
}
}
}
In the JSON representation of a Bake file, properties are objects, and attributes are values assigned to those objects.
The following example shows the same Bake file in the HCL format:
variable "TAG" {
default = "latest"
}
group "default" {
targets = ["webapp"]
}
target "webapp" {
dockerfile = "Dockerfile"
tags = ["docker.io/username/webapp:${TAG}"]
}
HCL is the preferred format for Bake files. Aside from syntactic differences, HCL lets you use features that the JSON and YAML formats don't support.
The examples in this document use the HCL format.
Target
A target reflects a single docker build
invocation.
Consider the following build command:
$ docker build \
--file=Dockerfile.webapp \
--tag=docker.io/username/webapp:latest \
https://github.com/username/webapp
You can express this command in a Bake file as follows:
target "webapp" {
dockerfile = "Dockerfile.webapp"
tags = ["docker.io/username/webapp:latest"]
context = "https://github.com/username/webapp"
}
The following table shows the complete list of attributes that you can assign to a target:
Name | Type | Description |
---|---|---|
args | Map | Build arguments |
annotations | List | Exporter annotations |
attest | List | Build attestations |
cache-from | List | External cache sources |
cache-to | List | External cache destinations |
context | String | Set of files located in the specified path or URL |
contexts | Map | Additional build contexts |
dockerfile-inline | String | Inline Dockerfile string |
dockerfile | String | Dockerfile location |
inherits | List | Inherit attributes from other targets |
labels | Map | Metadata for images |
matrix | Map | Define a set of variables that forks a target into multiple targets. |
name | String | Override the target name when using a matrix. |
no-cache-filter | List | Disable build cache for specific stages |
no-cache | Boolean | Disable build cache completely |
output | List | Output destinations |
platforms | List | Target platforms |
pull | Boolean | Always pull images |
secret | List | Secrets to expose to the build |
shm-size | List | Size of /dev/shm |
ssh | List | SSH agent sockets or keys to expose to the build |
tags | List | Image names and tags |
target | String | Target build stage |
ulimits | List | Ulimit options |
target.args
Use the args
attribute to define build arguments for the target.
This has the same effect as passing a
--build-arg
flag to the build command.
target "default" {
args = {
VERSION = "0.0.0+unknown"
}
}
You can set args
attributes to use null
values.
Doing so forces the target
to use the ARG
value specified in the Dockerfile.
variable "GO_VERSION" {
default = "1.20.3"
}
target "webapp" {
dockerfile = "webapp.Dockerfile"
tags = ["docker.io/username/webapp"]
}
target "db" {
args = {
GO_VERSION = null
}
dockerfile = "db.Dockerfile"
tags = ["docker.io/username/db"]
}
target.annotations
The annotations
attribute lets you add annotations to images built with bake.
The key takes a list of annotations, in the format of KEY=VALUE
.
target "default" {
output = ["type=image,name=foo"]
annotations = ["org.opencontainers.image.authors=dvdksn"]
}
is the same as
target "default" {
output = ["type=image,name=foo,annotation.org.opencontainers.image.authors=dvdksn"]
}
By default, the annotation is added to image manifests. You can configure the level of the annotations by adding a prefix to the annotation, containing a comma-separated list of all the levels that you want to annotate. The following example adds annotations to both the image index and manifests.
target "default" {
output = ["type=image,name=foo"]
annotations = ["index,manifest:org.opencontainers.image.authors=dvdksn"]
}
Read about the supported levels in Specifying annotation levels.
target.attest
The attest
attribute lets you apply
build attestations to the target.
This attribute accepts the long-form CSV version of attestation parameters.
target "default" {
attest = [
"type=provenance,mode=min",
"type=sbom"
]
}
target.cache-from
Build cache sources.
The builder imports cache from the locations you specify.
It uses the
Buildx cache storage backends,
and it works the same way as the
--cache-from
flag.
This takes a list value, so you can specify multiple cache sources.
target "app" {
cache-from = [
"type=s3,region=eu-west-1,bucket=mybucket",
"user/repo:cache",
]
}
target.cache-to
Build cache export destinations.
The builder exports its build cache to the locations you specify.
It uses the
Buildx cache storage backends,
and it works the same way as the
--cache-to
flag.
This takes a list value, so you can specify multiple cache export targets.
target "app" {
cache-to = [
"type=s3,region=eu-west-1,bucket=mybucket",
"type=inline"
]
}
target.context
Specifies the location of the build context to use for this target. Accepts a URL or a directory path. This is the same as the build context positional argument that you pass to the build command.
target "app" {
context = "./src/www"
}
This resolves to the current working directory ("."
) by default.
$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
"target": {
"default": {
"context": ".",
"dockerfile": "Dockerfile"
}
}
}
target.contexts
Additional build contexts.
This is the same as the
--build-context
flag.
This attribute takes a map, where keys result in named contexts that you can
reference in your builds.
You can specify different types of contexts, such local directories, Git URLs, and even other Bake targets. Bake automatically determines the type of a context based on the pattern of the context value.
Context type | Example |
---|---|
Container image | docker-image://alpine@sha256:0123456789 |
Git URL | https://github.com/user/proj.git |
HTTP URL | https://example.com/files |
Local directory | ../path/to/src |
Bake target | target:base |
Pin an image version
# docker-bake.hcl
target "app" {
contexts = {
alpine = "docker-image://alpine:3.13"
}
}
# Dockerfile
FROM alpine
RUN echo "Hello world"
Use a local directory
# docker-bake.hcl
target "app" {
contexts = {
src = "../path/to/source"
}
}
# Dockerfile
FROM scratch AS src
FROM golang
COPY --from=src . .
Use another target as base
Note
You should prefer to use regular multi-stage builds over this option. You can Use this feature when you have multiple Dockerfiles that can't be easily merged into one.
# docker-bake.hcl
target "base" {
dockerfile = "baseapp.Dockerfile"
}
target "app" {
contexts = {
baseapp = "target:base"
}
}
# Dockerfile
FROM baseapp
RUN echo "Hello world"
target.dockerfile-inline
Uses the string value as an inline Dockerfile for the build target.
target "default" {
dockerfile-inline = "FROM alpine\nENTRYPOINT [\"echo\", \"hello\"]"
}
The dockerfile-inline
takes precedence over the dockerfile
attribute.
If you specify both, Bake uses the inline version.
target.dockerfile
Name of the Dockerfile to use for the build.
This is the same as the
--file
flag for the docker build
command.
target "default" {
dockerfile = "./src/www/Dockerfile"
}
Resolves to "Dockerfile"
by default.
$ docker buildx bake --print -f - <<< 'target "default" {}'
[+] Building 0.0s (0/0)
{
"target": {
"default": {
"context": ".",
"dockerfile": "Dockerfile"
}
}
}
target.inherits
A target can inherit attributes from other targets.
Use inherits
to reference from one target to another.
In the following example,
the app-dev
target specifies an image name and tag.
The app-release
target uses inherits
to reuse the tag name.
variable "TAG" {
default = "latest"
}
target "app-dev" {
tags = ["docker.io/username/myapp:${TAG}"]
}
target "app-release" {
inherits = ["app-dev"]
platforms = ["linux/amd64", "linux/arm64"]
}
The inherits
attribute is a list,
meaning you can reuse attributes from multiple other targets.
In the following example, the app-release
target reuses attributes
from both the app-dev
and _release
targets.
target "app-dev" {
args = {
GO_VERSION = "1.20"
BUILDX_EXPERIMENTAL = 1
}
tags = ["docker.io/username/myapp"]
dockerfile = "app.Dockerfile"
labels = {
"org.opencontainers.image.source" = "https://github.com/username/myapp"
}
}
target "_release" {
args = {
BUILDKIT_CONTEXT_KEEP_GIT_DIR = 1
BUILDX_EXPERIMENTAL = 0
}
}
target "app-release" {
inherits = ["app-dev", "_release"]
platforms = ["linux/amd64", "linux/arm64"]
}
When inheriting attributes from multiple targets and there's a conflict,
the target that appears last in the inherits
list takes precedence.
The previous example defines the BUILDX_EXPERIMENTAL
argument twice for the app-release
target.
It resolves to 0
because the _release
target appears last in the inheritance chain:
$ docker buildx bake --print app-release
[+] Building 0.0s (0/0)
{
"group": {
"default": {
"targets": [
"app-release"
]
}
},
"target": {
"app-release": {
"context": ".",
"dockerfile": "app.Dockerfile",
"args": {
"BUILDKIT_CONTEXT_KEEP_GIT_DIR": "1",
"BUILDX_EXPERIMENTAL": "0",
"GO_VERSION": "1.20"
},
"labels": {
"org.opencontainers.image.source": "https://github.com/username/myapp"
},
"tags": [
"docker.io/username/myapp"
],
"platforms": [
"linux/amd64",
"linux/arm64"
]
}
}
}
target.labels
Assigns image labels to the build.
This is the same as the --label
flag for docker build
.
target "default" {
labels = {
"org.opencontainers.image.source" = "https://github.com/username/myapp"
"com.docker.image.source.entrypoint" = "Dockerfile"
}
}
It's possible to use a null
value for labels.
If you do, the builder uses the label value specified in the Dockerfile.
target.matrix
A matrix strategy lets you fork a single target into multiple different variants, based on parameters that you specify. This works in a similar way to [Matrix strategies for GitHub Actions]. You can use this to reduce duplication in your bake definition.
The matrix
attribute is a map of parameter names to lists of values.
Bake builds each possible combination of values as a separate target.
Each generated target must have a unique name.
To specify how target names should resolve, use the name
attribute.
The following example resolves the app
target to app-foo
and app-bar
.
It also uses the matrix value to define the
target build stage.
target "app" {
name = "app-${tgt}"
matrix = {
tgt = ["foo", "bar"]
}
target = tgt
}
$ docker buildx bake --print app
[+] Building 0.0s (0/0)
{
"group": {
"app": {
"targets": [
"app-foo",
"app-bar"
]
},
"default": {
"targets": [
"app"
]
}
},
"target": {
"app-bar": {
"context": ".",
"dockerfile": "Dockerfile",
"target": "bar"
},
"app-foo": {
"context": ".",
"dockerfile": "Dockerfile",
"target": "foo"
}
}
}
Multiple axes
You can specify multiple keys in your matrix to fork a target on multiple axes. When using multiple matrix keys, Bake builds every possible variant.
The following example builds four targets:
app-foo-1-0
app-foo-2-0
app-bar-1-0
app-bar-2-0
target "app" {
name = "app-${tgt}-${replace(version, ".", "-")}"
matrix = {
tgt = ["foo", "bar"]
version = ["1.0", "2.0"]
}
target = tgt
args = {
VERSION = version
}
}
Multiple values per matrix target
If you want to differentiate the matrix on more than just a single value, you can use maps as matrix values. Bake creates a target for each map, and you can access the nested values using dot notation.
The following example builds two targets:
app-foo-1-0
app-bar-2-0
target "app" {
name = "app-${item.tgt}-${replace(item.version, ".", "-")}"
matrix = {
item = [
{
tgt = "foo"
version = "1.0"
},
{
tgt = "bar"
version = "2.0"
}
]
}
target = item.tgt
args = {
VERSION = item.version
}
}
target.name
Specify name resolution for targets that use a matrix strategy.
The following example resolves the app
target to app-foo
and app-bar
.
target "app" {
name = "app-${tgt}"
matrix = {
tgt = ["foo", "bar"]
}
target = tgt
}
target.no-cache-filter
Don't use build cache for the specified stages.
This is the same as the --no-cache-filter
flag for docker build
.
The following example avoids build cache for the foo
build stage.
target "default" {
no-cache-filter = ["foo"]
}
target.no-cache
Don't use cache when building the image.
This is the same as the --no-cache
flag for docker build
.
target "default" {
no-cache = 1
}
target.output
Configuration for exporting the build output.
This is the same as the
--output
flag.
The following example configures the target to use a cache-only output,
target "default" {
output = ["type=cacheonly"]
}
target.platforms
Set target platforms for the build target.
This is the same as the
--platform
flag.
The following example creates a multi-platform build for three architectures.
target "default" {
platforms = ["linux/amd64", "linux/arm64", "linux/arm/v7"]
}
target.pull
Configures whether the builder should attempt to pull images when building the target.
This is the same as the --pull
flag for docker build
.
The following example forces the builder to always pull all images referenced in the build target.
target "default" {
pull = "always"
}
target.secret
Defines secrets to expose to the build target.
This is the same as the
--secret
flag.
variable "HOME" {
default = null
}
target "default" {
secret = [
"type=env,id=KUBECONFIG",
"type=file,id=aws,src=${HOME}/.aws/credentials"
]
}
This lets you mount the secret in your Dockerfile.
RUN --mount=type=secret,id=aws,target=/root/.aws/credentials \
aws cloudfront create-invalidation ...
RUN --mount=type=secret,id=KUBECONFIG \
KUBECONFIG=$(cat /run/secrets/KUBECONFIG) helm upgrade --install
target.shm-size
Sets the size of the shared memory allocated for build containers when using
RUN
instructions.
The format is <number><unit>
. number
must be greater than 0
. Unit is
optional and can be b
(bytes), k
(kilobytes), m
(megabytes), or g
(gigabytes). If you omit the unit, the system uses bytes.
This is the same as the --shm-size
flag for docker build
.
target "default" {
shm-size = "128m"
}
Note
In most cases, it is recommended to let the builder automatically determine the appropriate configurations. Manual adjustments should only be considered when specific performance tuning is required for complex build scenarios.
target.ssh
Defines SSH agent sockets or keys to expose to the build.
This is the same as the
--ssh
flag.
This can be useful if you need to access private repositories during a build.
target "default" {
ssh = ["default"]
}
FROM alpine
RUN --mount=type=ssh \
apk add git openssh-client \
&& install -m 0700 -d ~/.ssh \
&& ssh-keyscan github.com >> ~/.ssh/known_hosts \
&& git clone [email protected]:user/my-private-repo.git
target.tags
Image names and tags to use for the build target.
This is the same as the
--tag
flag.
target "default" {
tags = [
"org/repo:latest",
"myregistry.azurecr.io/team/image:v1"
]
}
target.target
Set the target build stage to build.
This is the same as the
--target
flag.
target "default" {
target = "binaries"
}
target.ulimits
Ulimits overrides the default ulimits of build's containers when using RUN
instructions and are specified with a soft and hard limit as such:
<type>=<soft limit>[:<hard limit>]
, for example:
target "app" {
ulimits = [
"nofile=1024:1024"
]
}
Note
If you do not provide a
hard limit
, thesoft limit
is used for both values. If noulimits
are set, they are inherited from the defaultulimits
set on the daemon.
Note
In most cases, it is recommended to let the builder automatically determine the appropriate configurations. Manual adjustments should only be considered when specific performance tuning is required for complex build scenarios.
Group
Groups allow you to invoke multiple builds (targets) at once.
group "default" {
targets = ["db", "webapp-dev"]
}
target "webapp-dev" {
dockerfile = "Dockerfile.webapp"
tags = ["docker.io/username/webapp:latest"]
}
target "db" {
dockerfile = "Dockerfile.db"
tags = ["docker.io/username/db"]
}
Groups take precedence over targets, if both exist with the same name.
The following bake file builds the default
group.
Bake ignores the default
target.
target "default" {
dockerfile-inline = "FROM ubuntu"
}
group "default" {
targets = ["alpine", "debian"]
}
target "alpine" {
dockerfile-inline = "FROM alpine"
}
target "debian" {
dockerfile-inline = "FROM debian"
}
Variable
The HCL file format supports variable block definitions. You can use variables as build arguments in your Dockerfile, or interpolate them in attribute values in your Bake file.
variable "TAG" {
default = "latest"
}
target "webapp-dev" {
dockerfile = "Dockerfile.webapp"
tags = ["docker.io/username/webapp:${TAG}"]
}
You can assign a default value for a variable in the Bake file,
or assign a null
value to it. If you assign a null
value,
Buildx uses the default value from the Dockerfile instead.
You can override variable defaults set in the Bake file using environment variables.
The following example sets the TAG
variable to dev
,
overriding the default latest
value shown in the previous example.
$ TAG=dev docker buildx bake webapp-dev
Built-in variables
The following variables are built-ins that you can use with Bake without having to define them.
Variable | Description |
---|---|
BAKE_CMD_CONTEXT | Holds the main context when building using a remote Bake file. |
BAKE_LOCAL_PLATFORM | Returns the current platform’s default platform specification (e.g. linux/amd64 ). |
Use environment variable as default
You can set a Bake variable to use the value of an environment variable as a default value:
variable "HOME" {
default = "$HOME"
}
Interpolate variables into attributes
To interpolate a variable into an attribute string value, you must use curly brackets. The following doesn't work:
variable "HOME" {
default = "$HOME"
}
target "default" {
ssh = ["default=$HOME/.ssh/id_rsa"]
}
Wrap the variable in curly brackets where you want to insert it:
variable "HOME" {
default = "$HOME"
}
target "default" {
- ssh = ["default=$HOME/.ssh/id_rsa"]
+ ssh = ["default=${HOME}/.ssh/id_rsa"]
}
Before you can interpolate a variable into an attribute, first you must declare it in the bake file, as demonstrated in the following example.
$ cat docker-bake.hcl
target "default" {
dockerfile-inline = "FROM ${BASE_IMAGE}"
}
$ docker buildx bake
[+] Building 0.0s (0/0)
docker-bake.hcl:2
--------------------
1 | target "default" {
2 | >>> dockerfile-inline = "FROM ${BASE_IMAGE}"
3 | }
4 |
--------------------
ERROR: docker-bake.hcl:2,31-41: Unknown variable; There is no variable named "BASE_IMAGE"., and 1 other diagnostic(s)
$ cat >> docker-bake.hcl
variable "BASE_IMAGE" {
default = "alpine"
}
$ docker buildx bake
[+] Building 0.6s (5/5) FINISHED
Function
A set of general-purpose functions provided by go-cty are available for use in HCL files:
# docker-bake.hcl
target "webapp-dev" {
dockerfile = "Dockerfile.webapp"
tags = ["docker.io/username/webapp:latest"]
args = {
buildno = "${add(123, 1)}"
}
}
In addition, user defined functions are also supported:
# docker-bake.hcl
function "increment" {
params = [number]
result = number + 1
}
target "webapp-dev" {
dockerfile = "Dockerfile.webapp"
tags = ["docker.io/username/webapp:latest"]
args = {
buildno = "${increment(123)}"
}
}
Note
See User defined HCL functions page for more details.