Fixed mysql tag dockerfile

Signed-off-by: Alessandro Tundo <alessandrotundo94@gmail.com>

mysql/Dockerfile

@@ -1,4 +1,4 @@
-FROM mysql:mysql:5.6
+FROM mysql:5.6
 
 MAINTAINER Mahmoud Zalt <mahmoud@zalt.me>
 
@@ -6,4 +6,4 @@ ADD my.cnf /etc/mysql/conf.d/my.cnf
 
 CMD ["mysqld"]
 
-EXPOSE 3306
+EXPOSE 3306