make build.context optional and default to .

[ndeloof]

What this PR does / why we need it:
make build.context optional and default to .

[glours]

LGTM

[milas]

LGTM!

This has been the behavior (unintentionally) of Compose v2.x all along, and nobody has complained about it...until we "fixed" it in v2.19, so I think this is a reasonable relaxing of strictness.

See also:

• loader: restore implicit default of . for build context compose-go#423
• loader: restore implicit default of . for build context with a warning compose-go#424

[danfundnel]

@ndeloof I may be wrong but it seems this change may have had some side-effects (not sure if intended) in the case where extends: is used to import compose specs from files in other dirs which specify a relative build.context path. Up until the 4.21 upgrade compose would take the reference in the included compose spec as relative to that file. It now seems to require it to be set relative to the directory docker-compose is called in (and, by the way, not the dir of the parent compose file using extends:, if that file is specified using -f). I think either behaviour is fine but this has broken the specs in our monorepo so I imagine others may also have to make this change.

[ndeloof]

@danfundnel I don't think so
quick experiment:
compose.yaml

services:
  app:
    extends:
      file: ./abstract/base.yaml
      service: base

base.yaml

services:
  base:
    build:
      context: ./base

$ docker compose config
services:
  app:
    build:
      context: /Users/nicolas/test/abstract/base
      dockerfile: Dockerfile