ci(docker): add v2 Dockerfile + GHCR publish job (#1646)

v1 published a container image but v2 arrived with no Dockerfile (the
client/server/cli layout is gone). Add a v2 image for the single-package /
launcher architecture and wire the release-gated GHCR publish job.

- `Dockerfile`: two-stage build — stage 1 installs + `npm pack`s the publishable
  tarball; stage 2 `npm install -g`s that tarball, so the image ships the exact
  same artifact as npm with a clean `mcp-inspector` bin. Defaults to `--web` on
  `0.0.0.0:6274`, browser auto-open off. `.dockerignore` keeps the context lean
  and forces a clean in-container install/build.
- `publish-github-container-registry` job in main.yml (ported from v1):
  release-gated, multi-arch (amd64/arm64) build+push to GHCR with a provenance
  attestation. Independent of the npm `publish` job.
- README: document the image + how to run it; the release section now covers
  both publish jobs.

Verified locally: `docker build` succeeds, the container serves `/` (HTTP 200
with the injected token) and dispatches `--cli` / `--tui`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BrihGWcrM9JGRyu41nzZYw

.github/workflows/main.yml

@@ -146,3 +146,57 @@ jobs:
         run: npm publish --access public --provenance
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
+
+  # Build and push the multi-arch container image to GHCR on a published
+  # release. The image installs the packed tarball (`Dockerfile`) so it ships
+  # the same artifact as npm. Independent of the npm `publish` job (both gated on
+  # the release event) — a container failure doesn't block the npm publish.
+  publish-github-container-registry:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'release'
+    environment: release
+    needs: build
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      id-token: write
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v6
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push Docker image
+        id: push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+
+      - name: Generate artifact attestation
+        uses: actions/attest-build-provenance@v3
+        with:
+          subject-name: ghcr.io/${{ github.repository }}
+          subject-digest: ${{ steps.push.outputs.digest }}
+          push-to-registry: true