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>

internal/api/v1/docker_test.go

@@ -0,0 +1,50 @@
+package v1
+
+import "testing"
+
+func TestParseDockerPath(t *testing.T) {
+	tests := []struct {
+		name      string
+		rest      string
+		wantOK    bool
+		wantImage string
+		wantKind  string
+		wantRef   string
+	}{
+		{"start upload trailing slash", "team/app/blobs/uploads/", true, "team/app", "upload", ""},
+		{"start upload no slash", "team/app/blobs/uploads", true, "team/app", "upload", ""},
+		{"patch upload with uuid", "team/app/blobs/uploads/abc-123", true, "team/app", "upload", "abc-123"},
+		{"single-segment image upload", "app/blobs/uploads/", true, "app", "upload", ""},
+		{"blob by digest", "team/app/blobs/sha256:deadbeef", true, "team/app", "blob", "sha256:deadbeef"},
+		{"manifest by tag", "team/app/manifests/v1.0.0", true, "team/app", "manifest", "v1.0.0"},
+		{"manifest by digest", "team/app/manifests/sha256:cafe", true, "team/app", "manifest", "sha256:cafe"},
+		{"tags list", "team/app/tags/list", true, "team/app", "tags", ""},
+		{"leading slash tolerated", "/team/app/manifests/latest", true, "team/app", "manifest", "latest"},
+		{"deep image name", "a/b/c/manifests/latest", true, "a/b/c", "manifest", "latest"},
+		{"unrecognised", "team/app/whatever", false, "", "", ""},
+		{"tags list without image", "tags/list", false, "", "", ""},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got, ok := parseDockerPath(tc.rest)
+			if ok != tc.wantOK {
+				t.Fatalf("ok = %v, want %v", ok, tc.wantOK)
+			}
+			if !tc.wantOK {
+				return
+			}
+			if got.image != tc.wantImage || got.kind != tc.wantKind || got.ref != tc.wantRef {
+				t.Fatalf("got %+v, want image=%q kind=%q ref=%q", got, tc.wantImage, tc.wantKind, tc.wantRef)
+			}
+		})
+	}
+}
+
+func TestIsDigest(t *testing.T) {
+	if !isDigest("sha256:abc") {
+		t.Fatal("sha256: prefix should be a digest")
+	}
+	if isDigest("v1.0.0") {
+		t.Fatal("a tag is not a digest")
+	}
+}