chore(docker): optimize builds and name images

- Add BuildKit cache mounts for NuGet and npm to persist package
  caches across builds
- Skip redundant restore on dotnet publish with --no-restore
- Add --prefer-offline to npm ci to prefer cached tarballs
- Tag images as randall/backend:latest and randall/frontend:latest
  via compose image: key

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cicd/docker/Dockerfile.frontend

@@ -4,7 +4,8 @@ WORKDIR /app
 
 # Install dependencies — copy lockfiles first to cache the npm layer
 COPY src/frontend/package.json src/frontend/package-lock.json ./
-RUN npm ci
+RUN --mount=type=cache,id=npm,target=/root/.npm \
+    npm ci --prefer-offline
 
 # Copy source
 COPY src/frontend/index.html ./