build(docker): warm prod cache at build time and add prod compose file

- Run cache:warmup in the build stage so containers start with a
  pre-built Symfony kernel and DI container
- Scope the cache volume to var/cache/prod/pools where Symfony writes
  pool data, preserving the warmed kernel across container restarts
- Add docker-compose.prod.yml for deploying the registry image without
  the dev override being picked up automatically
- Expand .dockerignore to exclude vendor/, tests/, docs, compose files,
  and env files from the build context

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

.dockerignore

@@ -1,6 +1,34 @@
+# Version control
 .git
 .gitea
+
+# AI / IDE tooling
+.claude
+
+# Dev tooling
+.gitignore
+/.phpunit.cache
+phpunit.xml.dist
+tests/
+
+# Documentation
+CHANGELOG.md
+CLAUDE.md
+README.md
+
+# Compose / deployment descriptors (not app code)
+docker-compose.yml
+docker-compose.override.yml
+docker-compose.prod.yml
+
+# Dependencies — re-installed from lockfile in the build stage;
+# a local vendor/ in the build context would silently override the clean install
+vendor/
+
+# Runtime dirs (generated at build or run time, not from source)
 var/
+
+# Env files — config is injected at runtime via Docker environment variables
+.env
 .env.local
 .env.*.local
-docker-compose.override.yml