Update mysql Docker tag to v5.7.26

docker-compose.yml

@@ -58,7 +58,7 @@ services:
   mysql:
     container_name: mysql
     # https://hub.docker.com/r/library/mysql/
-    image: mysql:5.7.25
+    image: mysql:5.7.26
     environment:
       - MYSQL_ALLOW_EMPTY_PASSWORD=true
       - MYSQL_DATABASE=treeherder