fix: resolve HEAD requests from metadata without streaming the body

Docker HEAD routes ran a full Fetch + io.Copy, downloading the whole blob
(and fetching upstream on a miss) only for net/http to discard the body.
Add Engine.Head, which answers cached artifacts/indexes from store
metadata and otherwise issues an upstream HEAD, and route HEAD to a
dedicated handler that writes headers only.

Refs #70

e2e/proxy_test.go

@@ -24,6 +24,39 @@ func TestProxyBlocklist(t *testing.T) {
 	assertStatus(t, apiURL("/api/v1/remote/blocklist-test/malware.exe"), http.StatusForbidden)
 }
 
+func TestProxyHeadBlocklist(t *testing.T) {
+	createRemote(t, `{
+		"name": "head-block-test",
+		"package_type": "generic",
+		"base_url": "https://example.com",
+		"blocklist": ["\\.exe$"],
+		"stale_on_error": true
+	}`)
+	defer deleteRemote(t, "head-block-test")
+
+	req, _ := http.NewRequest(http.MethodHead, apiURL("/v2/head-block-test/malware.exe"), nil)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("HEAD blocklisted path: got %d, want 403", resp.StatusCode)
+	}
+}
+
+func TestProxyHeadUnknownRemote(t *testing.T) {
+	req, _ := http.NewRequest(http.MethodHead, apiURL("/v2/nonexistent/some/path"), nil)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("HEAD: %v", err)
+	}
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusNotFound {
+		t.Fatalf("HEAD unknown remote: got %d, want 404", resp.StatusCode)
+	}
+}
+
 func TestProxyPatterns(t *testing.T) {
 	createRemote(t, `{
 		"name": "patterns-test",