feat: add Docker Registry V2 endpoint at /v2/ (#57)

The v3 Go rewrite removed the /v2/ Docker Registry compatibility endpoint. Docker clients need:
- GET/HEAD /v2/ → 200 (registry ping)
- GET/HEAD /v2/{remoteName}/* → proxy to the docker remote

Usage: `docker pull artifactapi.example.com/{remoteName}/image:tag`
Reviewed-on: #57
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/api/v1/proxy.go

@@ -37,6 +37,20 @@ func (h *ProxyHandler) Routes() chi.Router {
 	return r
 }
 
+func (h *ProxyHandler) DockerV2Routes() chi.Router {
+	r := chi.NewRouter()
+	r.Get("/", h.handleDockerPing)
+	r.Head("/", h.handleDockerPing)
+	r.Get("/{remoteName}/*", h.handleProxy)
+	r.Head("/{remoteName}/*", h.handleProxy)
+	return r
+}
+
+func (h *ProxyHandler) handleDockerPing(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Docker-Distribution-Api-Version", "registry/2.0")
+	w.WriteHeader(http.StatusOK)
+}
+
 func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 	remoteName := chi.URLParam(r, "remoteName")
 	path := chi.URLParam(r, "*")

internal/server/server.go

@@ -96,6 +96,7 @@ func (s *Server) routes() chi.Router {
 
 	proxyHandler := v1.NewProxyHandler(s.engine, s.virtEngine, s.db, s.store, s.localHandler)
 	r.Mount("/api/v1", proxyHandler.Routes())
+	r.Mount("/v2", proxyHandler.DockerV2Routes())
 
 	remotesHandler := v2.NewRemotesHandler(s.db)
 	virtualsHandler := v2.NewVirtualsHandler(s.db)