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.prod.yml

@@ -0,0 +1,32 @@
+services:
+  graph:
+    image: git.arthurerlich.de/haylan/git-contribution-graph:latest
+    container_name: git-contribution-graph
+    restart: unless-stopped
+    ports:
+      - "8080:8080"
+    environment:
+      APP_ENV: prod
+      APP_DEBUG: "0"
+      APP_SECRET: "${APP_SECRET}"
+      ALLOWED_HOSTS: "${ALLOWED_HOSTS:-}"
+      GITHUB_USER: "${GITHUB_USER:-}"
+      GITHUB_TOKEN: "${GITHUB_TOKEN:-}"
+      GITLAB_USER: "${GITLAB_USER:-}"
+      GITLAB_TOKEN: "${GITLAB_TOKEN:-}"
+      GITLAB_URL: "${GITLAB_URL:-}"
+      GITEA_USER: "${GITEA_USER:-}"
+      GITEA_TOKEN: "${GITEA_TOKEN:-}"
+      GITEA_URL: "${GITEA_URL:-}"
+    volumes:
+      - cache:/app/var/cache/prod/pools
+      - logs:/app/var/log
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+
+volumes:
+  cache:
+  logs: