Add support for labels during build

[ColinHebert]

Adds support for labels when building an images.
cf.
https://github.com/docker/docker-py/blob/0a97df1abc7236793b82041626088f54ad8d204f/docker/api/build.py#L94

Fixes#4038

[ColinHebert]

I'm not 100% sure that's the best way to write the test. I also used this in my dev environment to make sure it works:

docker-compose.yaml

version: '3.2'services:
web:
build:
context: .labels:
test: blahtest2: test

Dockerfile

FROM ubuntu

Execution

root# docker-compose build
Building web
Step 1/2 : FROM ubuntu
---> 0ef2e08ed3fa
Step 2/2 : LABEL "test" 'blah' "test2" 'test'
---> Using cache
---> 6c6c2d57f195
Successfully built 6c6c2d57f195

[shin-]

Thanks!

I think if you want to add something to the v3 format, you should submit a PR in the Engine repo first - they're the source of truth as far as v3 is concerned.

[ColinHebert]

Uh? That sounds a bit.. unwieldy.
In any case, I've opened moby/moby#32632 that updates the docker-compose schema.

[dnephin]

LGTM

[ColinHebert]

@dnephin@shin-, with the PR being approved (but not merged) on the moby/moby repo, what are the next steps to get this merged?

[ColinHebert]

@shin- would it be possible to get this one merged for the next release?

[shin-]

Ah yes, sorry - it just needs to be moved to the new 3.3 schema as per moby/moby#32972

[ColinHebert]

I see, how do you suggest we proceed?
There are other PRs that are attempting to modify the schema (see moby/moby#32743) are we importing the schema each time it's modified?

Also note that the schemas now live in docker/cli due to moby/moby#32694; see docker/cli#32 to get 3.3 in docker-cli

@shin-, @dnephin, should a different approach be devised here, where schemas would be held in a different repository which is "git submoduled" into both the cli and the compose project? To avoid divergence of the schemas between the two projects?
Simple changes like this one (it's literally two lines, one in the schema, one in service.py) should be easy to do. So far it's required 4 PRs across 3 different repository and even now, 23 days after the original PR was opened, it is still unclear how it's going to work :(

[shin-]

I don't think we are at a point where submodule would actually improve the workflow here. The reorganization at the engine level with the new Moby project and the docker/cli project split has caused this process to be a bit more involved than usual, but it's not the norm.

Either way: Merged via #4858