fixup: keep docker/*.conf and docker/entrypoint.sh in build context

Review feedback on PR #10: excluding the whole docker/ directory broke
frontend.prod.Dockerfile, which copies docker/nginx.conf and
docker/entrypoint.sh into the production nginx image.

- Replace docker/ with docker/*.Dockerfile so only the Dockerfiles are
  removed from the build context.
- Restore docker-compose*.yml exclusion.
- Correct the header comment to reflect that dev Dockerfiles COPY source
  subpaths, not the entire repo root.

Verified: docker compose -f docker-compose.prod.yml build frontend
succeeds and both COPY docker/... steps complete.

.dockerignore

@@ -1,6 +1,7 @@
 # Exclude everything that isn't strictly needed to build or run the dev images.
-# The dev Dockerfiles COPY . /app, so without this the image would bloat with
+# The dev Dockerfiles COPY source subpaths (frontend/, backend/, gradlew,
-# docs, scripts, git history, etc.
+# settings.gradle, etc.), so without this the image would bloat with docs,
+# scripts, git history, etc.
 
 # Build artifacts and caches (mounted as named volumes at runtime)
 .gradle
@@ -49,11 +50,13 @@ scripts/
 frontend/src/__tests__
 backend/src/test
 
-# Docker-related metadata (not needed inside the running image)
+# Docker metadata — Dockerfiles, .dockerignore, and compose files are not
+# needed inside the running image. Keep docker/*.conf and docker/entrypoint.sh
+# because frontend.prod.Dockerfile copies them into the production nginx image.
+docker/*.Dockerfile
 Dockerfile*
 .dockerignore
 docker-compose*.yml
-docker/
 
 # Misc
 *.log