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>

docker-compose.yml

@@ -21,7 +21,7 @@ services:
       GITEA_TOKEN: "${GITEA_TOKEN:-}"
       GITEA_URL: "${GITEA_URL:-}"
     volumes:
-      - cache:/app/var/cache
+      - cache:/app/var/cache/prod/pools
       - logs:/app/var/log
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8080/health"]