Update postgres in the Docker compose example to 12-alpine. (#7696)

changelog.d/7696.doc

@@ -0,0 +1 @@
+Update postgres image in example `docker-compose.yaml` to tag `12-alpine`.

contrib/docker/docker-compose.yml

@@ -50,7 +50,7 @@ services:
       - traefik.http.routers.https-synapse.tls.certResolver=le-ssl
 
   db:
-    image: docker.io/postgres:10-alpine
+    image: docker.io/postgres:12-alpine
     # Change that password, of course!
     environment:
       - POSTGRES_USER=synapse