feat: serve local docker repos as a real registry (#103)

## Why

Local `docker` repos had no write path — the `/v2` Docker Registry API only proxied to upstreams. This makes a local docker repo a genuine container 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.

## Changes

- Implement the Docker Registry HTTP API V2 read/write 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.

## Verification

- `scripts/docker-e2e.sh` passes, including the new `TestLocalDockerPushPull`.
- Verified a real end-to-end round-trip with skopeo against a live instance: pushed `hello-world`, pulled it back, loaded it into the docker daemon, and ran it successfully.

Reviewed-on: #103
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/charmbracelet/bubbletea v1.3.10
 	github.com/charmbracelet/lipgloss v1.1.0
 	github.com/go-chi/chi/v5 v5.3.0
+	github.com/google/uuid v1.6.0
 	github.com/jackc/pgx/v5 v5.10.0
 	github.com/minio/minio-go/v7 v7.2.0
 	github.com/redis/go-redis/v9 v9.20.0
@@ -46,7 +47,6 @@ require (
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-ole/go-ole v1.2.6 // indirect
-	github.com/google/uuid v1.6.0 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect