Change Container Image Names to FQDN in docker-compose examples

docker-compose.yml

@@ -20,7 +20,7 @@ services:
       # If you require an empty table prefix, use a space as the DB_TABLE_PREFIX
       # - "DB_TABLE_PREFIX= "
   lime-db:
-    image: mysql:5.7
+    image: docker.io/mysql:5.7
     environment:
       - "MYSQL_USER=limesurvey"
       - "MYSQL_DATABASE=limesurvey"