review: harden the Docker image + align GHCR action versions

Address PR #1648 review:

- Bump the GHCR action majors to match current v1 (docker/login-action v4,
  docker/metadata-action v6, docker/setup-buildx-action v4,
  actions/attest-build-provenance v4) to avoid Dependabot churn.
- Make the image tags explicit (`type=semver` + `flavor: latest=true`) instead
  of relying on `flavor.latest=auto`, so a release lands both the version tags
  and `:latest` — the README's bare `ghcr.io/…/inspector` URL won't 404.
- Run the container as the non-root `node` user with /home/node as workdir (its
  writable home is where the inspector stores the catalog / OAuth state).
- Add a HEALTHCHECK that probes `/` via Node's global fetch (no curl in slim).
- Trim specification/ and docs/ from the build context. (test-servers/ stays —
  `tsc -b` typechecks the integration tests that import its alias, so excluding
  it breaks the build.)

Re-verified locally: `docker build` succeeds, the container runs as uid 1000
(node), serves `/` (200), and the HEALTHCHECK reports healthy.

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

Dockerfile

@@ -31,6 +31,17 @@ ENV HOST=0.0.0.0 \
     MCP_AUTO_OPEN_ENABLED=false
 EXPOSE 6274
 
+# Run as the non-root `node` user the base image ships. Its home (/home/node) is
+# where the inspector writes runtime state (the default catalog, OAuth token
+# storage), so use it as the working directory.
+USER node
+WORKDIR /home/node
+
+# Report readiness by probing the served SPA (`/` needs no auth). Uses Node's
+# global fetch — no curl/wget in the slim image.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+    CMD node -e "fetch('http://127.0.0.1:'+(process.env.CLIENT_PORT||6274)+'/').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+
 # Default to the web UI; override the args to run --cli / --tui.
 ENTRYPOINT ["mcp-inspector"]
 CMD ["--web"]