useruid: drop unnecessary dockerfile:1.4 frontend directive (#48)

The generated uid_reconcile Dockerfile only uses ARG, FROM $ARG, USER,
COPY, and RUN — all handled by buildkit's built-in frontend. The
`# syntax=docker/dockerfile:1.4` declaration forced buildkit to pull
docker/dockerfile:* from a registry before parsing, which hangs
indefinitely in environments whose mirror routes docker.io through a
broken upstream (observed on DAP workspaces routing through an ECR
docker-hub mirror).

Adds a test guard so the directive can't be reintroduced.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bilby91

useruid.go

@@ -143,9 +143,15 @@ func statOwner(path string) (uid, gid int, ok bool) {
 // /etc/passwd and /etc/group directly via awk + sed so it works on
 // Debian (shadow tools) and Alpine/BusyBox (no shadow tools) alike —
 // no `usermod`/`groupmod`/`getent` runtime dependency.
+//
+// Intentionally no `# syntax=docker/dockerfile:X` directive: the
+// instructions used here (ARG, FROM $ARG, USER, COPY, RUN) are all
+// handled by buildkit's built-in frontend, and declaring an external
+// frontend forces buildkit to pull `docker/dockerfile:*` from a
+// registry before parsing — which hangs indefinitely in environments
+// whose registry mirror routes docker.io through a broken upstream.
 func generateUIDDockerfile(baseImage, user string, hostUID, hostGID int) string {
-	return "# syntax=docker/dockerfile:1.4\n" +
-		"ARG _DEV_CONTAINERS_BASE_IMAGE=" + baseImage + "\n" +
+	return "ARG _DEV_CONTAINERS_BASE_IMAGE=" + baseImage + "\n" +
 		"FROM $_DEV_CONTAINERS_BASE_IMAGE\n" +
 		"USER root\n" +
 		"ARG _REMOTE_USER=" + user + "\n" +

useruid_test.go

@@ -57,6 +57,13 @@ func TestGenerateUIDDockerfile_ContainsKeyDirectives(t *testing.T) {
 			t.Errorf("dockerfile missing %q\n--\n%s", want, df)
 		}
 	}
+	// Declaring an external dockerfile frontend forces buildkit to pull
+	// docker/dockerfile:* before parsing — which hangs in environments
+	// whose registry mirror routes docker.io through a broken upstream.
+	// Nothing in this Dockerfile needs a non-builtin frontend.
+	if strings.Contains(df, "# syntax=") {
+		t.Errorf("dockerfile must not declare a syntax= frontend; built-in frontend is sufficient\n--\n%s", df)
+	}
 }
 
 func TestUIDReconcileScript_PortableShape(t *testing.T) {