Make USER_UID and USER_GID configurable via environment variables

Allow overriding user and group IDs through environment variables with sensible defaults (1000), providing flexibility for different host system configurations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -9,8 +9,8 @@ services:
       redis:
         condition: service_started
     environment:
-      - USER_UID=1000
+      - USER_UID=${USER_UID:-1000}
-      - USER_GID=1000
+      - USER_GID=${USER_GID:-1000}
       - GITEA__database__DB_TYPE=postgres
       - GITEA__database__HOST=db:5432
       - GITEA__database__DATABASE=${POSTGRES_DATABASE}