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/docker-compose.yml

@@ -1,5 +1,6 @@
 services:
   backend:
+    image: randall/backend:latest
     build:
       context: ../..
       dockerfile: cicd/docker/Dockerfile.backend
@@ -20,6 +21,7 @@ services:
       start_period: 10s
 
   frontend:
+    image: randall/frontend:latest
     build:
       context: ../..
       dockerfile: cicd/docker/Dockerfile.frontend