Add new "ArchLastStageFrom" function for getting the effective "base" of the given image (needed for docs generation)

Author: tianon

go/src/bashbrew/docker.go

@@ -25,6 +25,15 @@ type dockerfileMetadata struct {
 	Froms  []string // every "FROM" or "COPY --from=xxx" value (minus named and/or numbered stages in the case of "--from=")
 }
 
+// this returns the "FROM" value for the last stage (which essentially determines the "base" for the final published image)
+func (r Repo) ArchLastStageFrom(arch string, entry *manifest.Manifest2822Entry) (string, error) {
+	dockerfileMeta, err := r.archDockerfileMetadata(arch, entry)
+	if err != nil {
+		return "", err
+	}
+	return dockerfileMeta.StageFroms[len(dockerfileMeta.StageFroms)-1], nil
+}
+
 func (r Repo) DockerFroms(entry *manifest.Manifest2822Entry) ([]string, error) {
 	return r.ArchDockerFroms(arch, entry)
 }