fix(dind): qualify single-segment refs with tag (alpine:3.20)

pkg/dind/registry.go

@@ -338,20 +338,32 @@ func tailHex(s string) string {
 // qualifyDockerHubRef ensures a reference carries an explicit registry
 // host. containerd's resolver treats the first path segment of an
 // unqualified reference as the hostname (so "ephpm/ephemerd:tag" → host
-// "ephpm" → DNS lookup fails). Docker CLI conventions default unqualified
-// refs to docker.io, prepending "library/" for single-segment names like
+// "ephpm" → DNS lookup fails; "alpine:3.20" → host "alpine", port "3.20"
+// → url.Parse rejects). Docker CLI conventions default unqualified refs
+// to docker.io, prepending "library/" for single-segment names like
 // "ubuntu". This helper applies the same rule.
+//
+// The slash is the disambiguator between a "host[:port]/path" reference
+// and a "name[:tag]" reference. With a slash, the part before it is a
+// host candidate and a dot or colon there means we've got a real
+// registry host (gcr.io, localhost:5000, my-registry.com:443, etc.).
+// Without a slash, there's no path and the whole string is a single-
+// segment image name with an optional tag — always docker.io/library/.
 func qualifyDockerHubRef(ref string) string {
-	first := ref
-	if i := strings.IndexByte(ref, '/'); i >= 0 {
-		first = ref[:i]
+	i := strings.IndexByte(ref, '/')
+	if i < 0 {
+		// No slash → single-segment name (possibly with tag/digest).
+		// Always qualifies with docker.io/library/. This covers the
+		// `alpine`, `alpine:3.20`, `alpine@sha256:...` cases.
+		return "docker.io/library/" + ref
 	}
+	first := ref[:i]
 	if strings.ContainsAny(first, ".:") || first == "localhost" {
+		// Looks like a real host (has a dot, or has a port, or is
+		// literally "localhost") — already qualified.
 		return ref
 	}
-	if !strings.Contains(ref, "/") {
-		return "docker.io/library/" + ref
-	}
+	// Multi-segment name without a host (e.g. "myorg/myimage[:tag]").
 	return "docker.io/" + ref
 }
 

pkg/dind/registry_helpers_test.go

@@ -80,11 +80,14 @@ func TestQualifyDockerHubRef(t *testing.T) {
 	}{
 		// Single-segment unqualified names get library/ prepended.
 		{"alpine", "docker.io/library/alpine"},
-		// Single-segment with tag — qualifyDockerHubRef sees the ":" and
-		// thinks "alpine:latest" is already a host, so it passes through
-		// unchanged. This is a known limitation; passing "alpine" without
-		// a tag is the supported form when callers want library/ added.
-		{"alpine:latest", "alpine:latest"},
+		// Single-segment WITH tag: the previous implementation bailed on the
+		// tag colon and passed it through unchanged, which made containerd's
+		// resolver try to parse "alpine" as a host and "3.20" as a port
+		// number. The disambiguator is the slash — no slash means no path,
+		// so the colon must be a tag separator.
+		{"alpine:latest", "docker.io/library/alpine:latest"},
+		{"alpine:3.20", "docker.io/library/alpine:3.20"},
+		{"alpine@sha256:abc123", "docker.io/library/alpine@sha256:abc123"},
 		// Two-segment names get docker.io/ prepended.
 		{"myorg/myimage", "docker.io/myorg/myimage"},
 		{"myorg/myimage:tag", "docker.io/myorg/myimage:tag"},