bake: fix BAKE_CMD_CONTEXT relative path resolution

[jedevc]

🛠️ Fixes path resolution issue with BAKE_CMD_CONTEXT.

In https://docs.docker.com/build/bake/remote-definition/, the following snippet no longer works:

$ docker buildx bake "https://github.com/tonistiigi/buildx.git#remote-test" --print
{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "dockerfile-inline": "FROM alpine\nWORKDIR /src\nCOPY . .\nRUN ls -l \u0026\u0026 stop\n"
    }
  }
}

Instead returning the following error:

$ docker buildx bake "https://github.com/tonistiigi/buildx.git#remote-test" --print
ERROR: Rel: can't make . relative to /home/jedevc/Documents/Docker/buildx

This issue comes from

buildx/bake/bake.go

Lines 1007 to 1010 in 341fb65

	rel, err := filepath.Rel(wd, p)
	if err != nil {
	return err
	}

To resolve this issue, we need to ensure that the passed path is an absolute path, instead of the relative path ..

To prevent further regression, I add a couple of tests to build against a remote git repo (which is where most of the code in this PR comes from 😱).

cc @crazy-max @dvdksn

[crazy-max]

Can we have a test case for

buildx/bake/bake.go

Lines 1011 to 1013 in 7cef021

	if strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
	return errors.Errorf("path %s is outside of the working directory, please set BAKE_ALLOW_REMOTE_FS_ACCESS=1", p)
	}

?

[jedevc]

Can we have a test case

@crazy-max sure, will work on this.

[tonistiigi]

Iiuc the util/gitutil serves dual purposes atm and is used for reading VCS info for builds as well as test utilities. This isn't ideal and these responsibilities should be split. Not needed in this PR though.