Proposal: new build config section

[dnephin]

Motivation for this proposal

• Support for passing build-time variables in build context moby/moby#15182 will be in 1.9.0 allow for build args
• new users are often confused when the environment section isn't applied during the build phase

Proposal

Move all build related configuration into a build: section in the service. Example

web:
  build:
    context: .
    dockerfile: Dockerfile.name
    args:
       key: value
  ...

This should help clarify the fields that are related to the build phase (and the fields that are not).

Backwards compatibility

On the first release both build and dockerfile would still be allowed at the top level of the service definition. A dockerfile at the top level would print a deprecation warning.

On the following release dockerfile at the top level of the service definition would be removed, and it must be under the build section. For trivial cases, a config of build: path/to/context would still be supported.

Related #2111

[thaJeztah]

I like this approach, sgtm. For the deprecation of the top-level fields; perhaps Compose should add a deprecation policy similar to Engine; https://github.com/docker/docker/blob/master/docs/misc/deprecated.md. This gives users time to adapt, and a clear notice when a feature will be removed,

[blaggacao]

+1 can't wait... 💃

[mosop]

👍

[marcellodesales]

👍

[lechen26]

is this option was added to docker-compose? the support for build-args?

[Silex]

👍