Consider generated .dockerignore support for AddNodeApp Dockerfiles

AddBunApp now emits generated .dockerignore content via BuildKit's per-Dockerfile convention so generated Dockerfiles can safely use COPY . . without pulling local node_modules, .git, dotenv files, or other local artifacts into the build context.\n\nAddNodeApp's generated Dockerfile also copies the app source from the build context and may benefit from the same default BuildContextIgnoreContent behavior for parity. This should be evaluated separately because Node apps may have established expectations around existing Dockerfile generation and package-manager workflows.\n\nFollow-up from PR #17416.