add build instructions to prod docker-compose

docker-compose.prod.yml

@@ -1,6 +1,9 @@
 services:
   web:
     image: eitchtee/wygiwyh:latest
+    build:
+      context: .
+      dockerfile: ./docker/prod/django/Dockerfile
     container_name: ${SERVER_NAME}
     ports:
       - "${OUTBOUND_PORT:-8000}:${INTERNAL_PORT:-8000}"