Add option to serve local archive files

Set an environment variable in the docker compose file, then reference
that variable in main.py to mount the local archive so that the links
generated by auto-archiver will work correctly. Fixes #8

docker-compose.yml

@@ -14,6 +14,7 @@ services:
     environment:
       - CELERY_BROKER_URL=redis://:${REDIS_PASSWORD}@redis:6379/0
       - CELERY_RESULT_BACKEND=redis://:${REDIS_PASSWORD}@redis:6379/0
+      - SERVE_LOCAL_ARCHIVE=/app/local_archive # See orchestration.yaml local_storage.save_to
     depends_on:
       - redis