Fix Docker build: lockfile, BuildKit DNS, and setuptools build backend

- Generate frontend/package-lock.json (required by npm ci)
- Add network: host to BuildKit build stages to fix DNS in pip installs
- Switch pyproject.toml build backend to setuptools.build_meta (stable)

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

backend/pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
-requires = ["setuptools"]
-build-backend = "setuptools.backends.legacy:build"
+requires = ["setuptools>=45"]
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "destroying_sap"