feat: serve local docker repos as a real registry

Local docker repos previously had no write path — the /v2 Docker Registry
API only proxied to upstreams. This makes a local docker repo a genuine
registry so `docker push`/`docker pull` (and podman/skopeo/buildah) work
against it directly, matching the project principle that a local repo is the
real thing rather than a mirror.

- Implement the Docker Registry HTTP API V2 write/read half for local docker
  repos: blob uploads (monolithic and chunked POST/PATCH/PUT), manifest push,
  tags list, and blob/manifest GET/HEAD.
- Store blobs and manifests through the existing content-addressable store;
  keep a local_files reference per (repo, image) so the GC does not reap them.
  Tags are mutable (UpsertLocalFile); digests and blobs are immutable.
- Dispatch /v2 reads to the local handler for local docker repos and fall
  through to the upstream proxy otherwise; writes are local-docker only.
- Add UpsertLocalFile for mutable tag references.
- Cover the push/pull round-trip with a dockerised e2e test and unit-test the
  registry path parser. Document the registry in the README.

internal/api/v1/proxy.go

@@ -23,10 +23,20 @@ type ProxyHandler struct {
 	db            *database.DB
 	store         *storage.S3
 	local         *v2.LocalHandler
+	cas           *storage.CAS
+	uploads       *uploadStore
 }
 
 func NewProxyHandler(engine *proxy.Engine, virtualEngine *virtual.Engine, db *database.DB, store *storage.S3, local *v2.LocalHandler) *ProxyHandler {
-	return &ProxyHandler{engine: engine, virtualEngine: virtualEngine, db: db, store: store, local: local}
+	return &ProxyHandler{
+		engine:        engine,
+		virtualEngine: virtualEngine,
+		db:            db,
+		store:         store,
+		local:         local,
+		cas:           storage.NewCAS(store),
+		uploads:       newUploadStore(),
+	}
 }
 
 func (h *ProxyHandler) Routes() chi.Router {
@@ -37,12 +47,20 @@ func (h *ProxyHandler) Routes() chi.Router {
 	return r
 }
 
+// DockerV2Routes mounts the Docker Registry HTTP API V2. Reads (GET/HEAD)
+// dispatch to a local registry implementation for local docker repos and fall
+// through to the upstream proxy otherwise; writes (POST/PATCH/PUT/DELETE) are
+// only valid for local docker repos and drive push.
 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.handleProxyHead)
+	r.Get("/{remoteName}/*", h.dockerGet)
+	r.Head("/{remoteName}/*", h.dockerHead)
+	r.Post("/{remoteName}/*", h.dockerPost)
+	r.Patch("/{remoteName}/*", h.dockerPatch)
+	r.Put("/{remoteName}/*", h.dockerPut)
+	r.Delete("/{remoteName}/*", h.dockerDelete)
 	return r
 }