Compose file build reference

Estimated reading time: 10 minutes

Compose specification is a platform-neutral way to define multi-container applications. A Compose implementation focusing on development use-case to run application on local machine will obviously also support (re)building application from sources. The Compose Build specification allows to define the build process within a Compose file in a portable way.

Definitions

Compose Specification is extended to support an OPTIONAL build subsection on services. This section define the build requirements for service container image. Only a subset of Compose file services MAY define such a Build subsection, others being created based on Image attribute. When a Build subsection is present for a service, it is valid for a Compose file to miss an Image attribute for corresponding service, as Compose implementation can build image from source.

Build can be either specified as a single string defining a context path, or as a detailed build definition.

In the former case, the whole path is used as a Docker context to execute a docker build, looking for a canonical Dockerfile at context root. Context path can be absolute or relative, and if so relative path MUST be resolved from Compose file parent folder. As an absolute path prevent the Compose file to be portable, Compose implementation SHOULD warn user accordingly.

In the later case, build arguments can be specified, including an alternate Dockerfile location. This one can be absolute or relative path. If Dockerfile path is relative, it MUST be resolved from context path. As an absolute path prevent the Compose file to be portable, Compose implementation SHOULD warn user if an absolute alternate Dockerfile path is used.

Consistency with Image

When service definition do include both Image attribute and a Build section, Compose implementation can’t guarantee a pulled image is strictly equivalent to building the same image from sources. Without any explicit user directives, Compose implementation with Build support MUST first try to pull Image, then build from source if image was not found on registry. Compose implementation MAY offer options to customize this behaviour by user request.

Publishing built images

Compose implementation with Build support SHOULD offer an option to push built images to a registry. Doing so, it MUST NOT try to push service images without an Image attribute. Compose implementation SHOULD warn user about missing Image attribute which prevent image being pushed.

Compose implementation MAY offer a mechanism to compute an Image attribute for service when not explicitly declared in yaml file. In such a case, the resulting Compose configuration is considered to have a valid Image attribute, whenever the actual raw yaml file doesn’t explicitly declare one.

Illustrative sample

The following sample illustrates Compose specification concepts with a concrete sample application. The sample is non-normative.

services:
  frontend:
    image: awesome/webapp
    build: ./webapp

  backend:
    image: awesome/database
    build:
      context: backend
      dockerfile: ../backend.Dockerfile

  custom:
    build: ~/custom

When used to build service images from source, such a Compose file will create three docker images:

  • awesome/webapp docker image is built using webapp sub-directory within Compose file parent folder as docker build context. Lack of a Dockerfile within this folder will throw an error.
  • awesome/database docker image is built using backend sub-directory within Compose file parent folder. backend.Dockerfile file is used to define build steps, this file is searched relative to context path, which means for this sample .. will resolve to Compose file parent folder, so backend.Dockerfile is a sibling file.
  • a docker image is built using custom directory within user’s HOME as docker context. Compose implementation warn user about non-portable path used to build image.

On push, both awesome/webapp and awesome/database docker images are pushed to (default) registry. custom service image is skipped as no Image attribute is set and user is warned about this missing attribute.

Build definition

The build element define configuration options that are applied by Compose implementations to build Docker image from source. build can be specified either as a string containing a path to the build context or a detailed structure:

services:
  webapp:
    build: ./dir

Using this string syntax, only the build context can be configured as a relative path to the Compose file’s parent folder. This path MUST be a directory and contain a Dockerfile.

Alternatively build can be an object with fields defined as follow

context (REQUIRED)

context defines either a path to a directory containing a Dockerfile, or a url to a git repository.

When the value supplied is a relative path, it MUST be interpreted as relative to the location of the Compose file. Compose implementations MUST warn user about absolute path used to define build context as those prevent Compose file from being portable.

build:
  context: ./dir

dockerfile

dockerfile allows to set an alternate Dockerfile. A relative path MUST be resolved from the build context. Compose implementations MUST warn user about absolute path used to define Dockerfile as those prevent Compose file from being portable.

build:
  context: .
  dockerfile: webapp.Dockerfile

args

args define build arguments, i.e. Dockerfile ARG values.

Using following Dockerfile:

ARG GIT_COMMIT
RUN echo "Based on commit: $GIT_COMMIT"

args can be set in Compose file under the build key to define GIT_COMMIT. args can be set a mapping or a list:

build:
  context: .
  args:
    GIT_COMMIT: cdc3b19
build:
  context: .
  args:
    - GIT_COMMIT=cdc3b19

Value can be omitted when specifying a build argument, in which case its value at build time MUST be obtained by user interaction, otherwise build arg won’t be set when building the Docker image.

args:
  - GIT_COMMIT

ssh

ssh defines SSH authentications that the image builder SHOULD use during image build (e.g., cloning private repository)

ssh property syntax can be either:

  • default - let the builder connect to the ssh-agent.
  • ID=path - a key/value definition of an ID and the associated path. Can be either a PEM file, or path to ssh-agent socket

Simple default sample

build:
  context: .
  ssh: 
    - default   # mount the default ssh agent

or

build:
  context: .
  ssh: ["default"]   # mount the default ssh agent

Using a custom id myproject with path to a local SSH key:

build:
  context: .
  ssh: 
    - myproject=~/.ssh/myproject.pem

Image builder can then rely on this to mount SSH key during build. For illustration, BuildKit extended syntax can be used to mount ssh key set by ID and access a secured resource:

RUN --mount=type=ssh,id=myproject git clone ...

cache_from

cache_from defines a list of sources the Image builder SHOULD use for cache resolution.

Cache location syntax MUST follow the global format [NAME|type=TYPE[,KEY=VALUE]]. Simple NAME is actually a shortcut notation for type=registry,ref=NAME.

Compose Builder implementations MAY support custom types, the Compose Specification defines canonical types which MUST be supported:

  • registry to retrieve build cache from an OCI image set by key ref
build:
  context: .
  cache_from:
    - alpine:latest
    - type=local,src=path/to/cache
    - type=gha

Unsupported caches MUST be ignored and not prevent user from building image.

cache_to

cache_to defines a list of export locations to be used to share build cache with future builds.

build:
  context: .
  cache_to: 
   - user/app:cache
   - type=local,dest=path/to/cache

Cache target is defined using the same type=TYPE[,KEY=VALUE] syntax defined by cache_from.

Unsupported cache target MUST be ignored and not prevent user from building image.

extra_hosts

extra_hosts adds hostname mappings at build-time. Use the same syntax as extra_hosts.

extra_hosts:
  - "somehost:162.242.195.82"
  - "otherhost:50.31.209.229"

Compose implementations MUST create matching entry with the IP address and hostname in the container’s network configuration, which means for Linux /etc/hosts will get extra lines:

162.242.195.82  somehost
50.31.209.229   otherhost

isolation

isolation specifies a build’s container isolation technology. Like isolation supported values are platform-specific.

labels

labels add metadata to the resulting image. labels can be set either as an array or a map.

reverse-DNS notation SHOULD be used to prevent labels from conflicting with those used by other software.

build:
  context: .
  labels:
    com.example.description: "Accounting webapp"
    com.example.department: "Finance"
    com.example.label-with-empty-value: ""
build:
  context: .
  labels:
    - "com.example.description=Accounting webapp"
    - "com.example.department=Finance"
    - "com.example.label-with-empty-value"

shm_size

shm_size set the size of the shared memory (/dev/shm partition on Linux) allocated for building Docker image. Specify as an integer value representing the number of bytes or as a string expressing a byte value.

build:
  context: .
  shm_size: '2gb'
build:
  context: .
  shm_size: 10000000

target

target defines the stage to build as defined inside a multi-stage Dockerfile.

build:
  context: .
  target: prod

Implementations

fig, composition, compose, docker