fix(ko): Default repo for pushing ko:// images

When using the `ko://` scheme prefix for the Skaffold image name, the
rest of the image name, following the prefix, is a Go import path.

This import path maps to the Git repository (typically), and won't match
an image registry name (e.g., `ko://github.com/org/repo` vs
`gcr.io/project_id`).

With this change, if _all_ of the following are true, the build results
in an error that instructs the user to set the default repo:

- the image name in `skaffold.yaml` uses the `ko://` prefix; and
- the Skaffold default repo is _not_ set; and
- the image is being pushed to a registry (rather than sideloaded to a
  Docker daemon).

Fixes: #6933
Tracking: #6041

pkg/skaffold/build/ko/build.go

@@ -37,6 +37,9 @@ import (
 // identifier. The image identifier is the tag or digest for pushed images, or
 // the docker image ID for sideloaded images.
 func (b *Builder) Build(ctx context.Context, out io.Writer, a *latestV1.Artifact, ref string) (string, error) {
+	if b.pushImages && strings.HasPrefix(ref, build.StrictScheme) {
+		return "", fmt.Errorf("default repo must be set when using the 'ko://' prefix and pushing to a registry: %s, see https://skaffold.dev/docs/environment/image-registries/", a.ImageName)
+	}
 	koBuilder, err := b.newKoBuilder(ctx, a)
 	if err != nil {
 		return "", fmt.Errorf("error creating ko builder: %w", err)

pkg/skaffold/build/ko/build_test.go

@@ -40,6 +40,7 @@ func TestBuild(t *testing.T) {
 		pushImages              bool
 		imageRef                string
 		expectedImageIdentifier string
+		shouldErr               bool
 	}{
 		{
 			description:             "pushed image with tag",
@@ -53,6 +54,12 @@ func TestBuild(t *testing.T) {
 			imageRef:                "registry.example.com/repo/image2:any",
 			expectedImageIdentifier: "ab737430e80b",
 		},
+		{
+			description: "error for missing default repo when using ko:// prefix combined with pushing image to a registry",
+			pushImages:  true,
+			imageRef:    "ko://github.com/google/ko",
+			shouldErr:   true,
+		},
 	}
 	for _, test := range tests {
 		testutil.Run(t, test.description, func(t *testutil.T) {
@@ -66,7 +73,7 @@ func TestBuild(t *testing.T) {
 				ImageName: importPath,
 			}
 			gotImageIdentifier, err := b.Build(context.Background(), nil, artifact, test.imageRef)
-			t.CheckNoError(err)
+			t.CheckErrorAndFailNow(test.shouldErr, err)
 			t.CheckDeepEqual(test.expectedImageIdentifier, gotImageIdentifier)
 		})
 	}