docker-compose: Support absolute path to Dockerfile in docker-compose build

[meshulam]

The Docker Compose V2 beta (enabled by default in the latest Docker for Mac) resolves absolute paths for build Dockerfiles in the output of docker-compose config.

Old behavior:

❯ cat docker-compose.yml
version: '3'
services:
  foo:
    build:
      context: ../foo
      dockerfile: Dockerfile

❯ docker-compose disable-v2
❯ docker-compose version
docker-compose version 1.29.2, build 5becea4c
docker-py version: 5.0.0
CPython version: 3.9.0
OpenSSL version: OpenSSL 1.1.1h  22 Sep 2020
❯ docker-compose config
services:
  foo:
    build:
      context: /Users/mattm/dev/foo
      dockerfile: Dockerfile
version: '3'

New behavior:

❯ docker-compose enable-v2
❯ docker-compose version
Docker Compose version v2.0.0-beta.6
❯ docker-compose config
services:
  foo:
    build:
      context: /Users/mattm/dev/foo
      dockerfile: /Users/mattm/dev/foo/Dockerfile
    networks:
      default: null
networks:
  default:
    name: tilt_default

This was always allowed according to the Compose File spec, but Tilt assumes the Dockerfile is relative and unconditionally joins it to the context dir.

This PR adds support for absolute paths to Dockerfiles, whether explicitly configured in the docker-compose.yml or as emitted by Compose V2.

[ebenoist]

@meshulam thanks for doing this!

[nicks]

LGTM, thanks!