URL-Encode password in broker_url

Needed to prevent connection errors if the password contains non-urlsafe characters.

Dockerfile change because all the services using it are mirroring ./ in /app now anyway.
Flower needs because it needs bookwyrm in order to read the celerywyrm config (because flower only allows overwriting broker_url from the command line and we want to do it in Python so we have access to a urlencode function).

docker-compose.yml

@@ -74,10 +74,10 @@ services:
     restart: on-failure
   flower:
     build: .
-    command: flower --port=${FLOWER_PORT}
+    command: flower -A celerywyrm
     env_file: .env
-    environment:
-      - CELERY_BROKER_URL=${CELERY_BROKER}
+    volumes:
+      - .:/app
     networks:
       - main
     depends_on: