refactor: mount source and plugins instead of baking into image

Containerfile now installs only dependencies (maxminddb). Source
code, plugins, and config are volume-mounted via docker-compose.
Code changes no longer require an image rebuild.

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

docker-compose.yml

@@ -6,6 +6,7 @@ services:
     container_name: derp
     restart: unless-stopped
     volumes:
-      - ./config/derp.toml:/app/config/derp.toml:ro,Z
+      - ./src:/app/src:ro,Z
       - ./plugins:/app/plugins:ro,Z
+      - ./config/derp.toml:/app/config/derp.toml:ro,Z
     command: ["--verbose"]