Specify the Docker registry for the postgres image

contrib/docker/docker-compose.yml

@@ -35,7 +35,7 @@ services:
       - traefik.port=8448
 
   db:
-    image: postgres:10-alpine
+    image: docker.io/postgres:10-alpine
     # Change that password, of course!
     environment:
       - POSTGRES_USER=synapse