bake: add file-relative path opt-in

[crazy-max]

fixes #1028
fixes #197

This adds an opt-in Bake mode that resolves local directory paths in target.context and target.contexts relative to the directory of the first Bake file when BUILDX_BAKE_FILE_RELATIVE_PATHS=1 is set.

Bake now accepts a parser option that rebases local directory build contexts after HCL or Compose files are parsed. The buildx bake command enables this option from BUILDX_BAKE_FILE_RELATIVE_PATHS, while cwd:// remains available for paths that should stay relative to the invocation directory.

[crazy-max]

Needs docs follow-up: https://docs.docker.com/build/building/variables/#build-tool-configuration-variables

[tonistiigi]

Looks like default . does not atm take the relative mode into account

	t.Run("default-context-in-subdirectory", func(t *testing.T) {
		bakefile := []byte(`
target "default" {
  dockerfile-inline = <<EOT
FROM scratch
COPY marker /marker
EOT
}
`)

		dir := tmpdir(
			t,
			fstest.CreateDir("definitions", 0700),
			fstest.CreateFile("definitions/docker-bake.hcl", bakefile, 0600),
			fstest.CreateFile("definitions/marker", []byte("marker"), 0600),
		)
		dirDest := t.TempDir()

		out, err := bakeCmd(
			sb,
			withDir(dir),
			withArgs("--file", "definitions/docker-bake.hcl", "--set", "*.output=type=local,dest="+dirDest),
			withEnv("BUILDX_BAKE_FILE_RELATIVE_PATHS=1"),
		)
		require.NoError(t, err, out)
		require.FileExists(t, filepath.Join(dirDest, "marker"))
	})

[tonistiigi]

Why the "first" bake file? Rather than "current" bake file. Even if this is more complicated, I think that should be the difference as the benefit of this method is that there isn't one global target directory.