Update Compose Spec to v2

[ncucinotta]

To support the latest Composefile syntax features. More specifically, I'm interested in supporting env_files being optional:

env_file:
  - path: ...
    required: ...

which currently crashes with:

Error: failed to setup new daemon: failed to parse docker compose file: validating /etc/docker/compose/docker-compose.yml: services.my_service.env_file must be a string
Usage:
  blazar run [flags]
Flags:
  -h, --help   help for run
Global Flags:
      --config string   config toml file

V2 has been chosen since the spec update I required first landed in v2.0.0-beta.2. Finally, instead of refactoring the entire codebase adding Context support to the functions working with Composefiles, I opted to use context.Background(), which was also the default fallback when first adding Context support upstream (compose-spec/compose-go@ad9be94).

[qezz]

thanks for looking into it

1/5: maybe we can throw in a test for the env_file, just a simple one