Support --tag and --tag-only with nop publisher (#797)

* Support --tag and --tag-only with nop publisher

* log the output, for debugging

* unset KO_DOCKER_REPO for push=false test

* run e2e test first before other stuff

* review feedback

.github/workflows/e2e.yaml

@@ -33,6 +33,11 @@ jobs:
         # cribbed from https://gist.github.com/Syeberman/39d81b1e17d091be5657ecd6fbff0753
         eval $(go env | sed -r 's/^(set )?(\w+)=("?)(.*)\3$/\2="\4"/gm')
 
+        # Check that building without push prints the tag (and sha)
+        KO_DOCKER_REPO="" go run ./ build --push=false ./test | grep ":latest@sha256:"
+        KO_DOCKER_REPO="" go run ./ build --push=false -t test ./test | grep ":test@sha256:"
+        KO_DOCKER_REPO="" go run ./ build --push=false -t test --tag-only ./test | grep ":test$"
+
         export PLATFORM=${GOOS}/${GOARCH}
 
         if [[ "${{ matrix.platform }}" == "windows-latest" ]]; then

pkg/commands/resolver.go

@@ -224,9 +224,16 @@ func makePublisher(po *options.PublishOptions) (publish.Interface, error) {
 		// If not publishing, at least generate a digest to simulate
 		// publishing.
 		if len(publishers) == 0 {
+			// If one or more tags are specified, use the first tag in the list
+			var tag string
+			if len(po.Tags) >= 1 {
+				tag = po.Tags[0]
+			}
 			publishers = append(publishers, nopPublisher{
 				repoName: repoName,
 				namer:    namer,
+				tag:      tag,
+				tagOnly:  po.TagOnly,
 			})
 		}
 
@@ -256,15 +263,27 @@ func makePublisher(po *options.PublishOptions) (publish.Interface, error) {
 type nopPublisher struct {
 	repoName string
 	namer    publish.Namer
+	tag      string
+	tagOnly  bool
 }
 
 func (n nopPublisher) Publish(_ context.Context, br build.Result, s string) (name.Reference, error) {
 	s = strings.TrimPrefix(s, build.StrictScheme)
+	nm := n.namer(n.repoName, s)
+	if n.tagOnly {
+		if n.tag == "" {
+			return nil, errors.New("must specify tag if requesting tag only")
+		}
+		return name.NewTag(fmt.Sprintf("%s:%s", nm, n.tag))
+	}
 	h, err := br.Digest()
 	if err != nil {
 		return nil, err
 	}
-	return name.NewDigest(fmt.Sprintf("%s@%s", n.namer(n.repoName, s), h))
+	if n.tag == "" {
+		return name.NewDigest(fmt.Sprintf("%s@%s", nm, h))
+	}
+	return name.NewDigest(fmt.Sprintf("%s:%s@%s", nm, n.tag, h))
 }
 
 func (n nopPublisher) Close() error { return nil }