Forced use of centos 6.

Since the release of centos 7, this container is no longer building correctly:

    Step 8 : RUN yum -y -q localinstall --nogpgcheck /tmp/zabbix-server-mysql-2.4.0-1.el6.x86_64.rpm
     ---> Running in 098be717ae78
    Error: Package: iksemel-1.4-2.el6.x86_64 (epel)
               Requires: libgnutls.so.26(GNUTLS_1_4)(64bit)
    Error: Package: zabbix-server-mysql-2.4.0-1.el6.x86_64 (/zabbix-server-mysql-2.4.0-1.el6.x86_64)
               Requires: libnetsnmp.so.20()(64bit)
    Error: Package: zabbix-server-mysql-2.4.0-1.el6.x86_64 (/zabbix-server-mysql-2.4.0-1.el6.x86_64)
               Requires: libmysqlclient.so.16()(64bit)
    Error: Package: zabbix-server-mysql-2.4.0-1.el6.x86_64 (/zabbix-server-mysql-2.4.0-1.el6.x86_64)
               Requires: libmysqlclient.so.16(libmysqlclient_16)(64bit)
    Error: Package: iksemel-1.4-2.el6.x86_64 (epel)
               Requires: libgnutls.so.26()(64bit)
     You could try using --skip-broken to work around the problem
    ** Found 1 pre-existing rpmdb problem(s), 'yum check' output follows:
    fakesystemd-1-17.el7.centos.noarch has installed conflicts systemd: fakesystemd-1-17.el7.centos.noarch
    2014/11/13 08:36:09 The command [/bin/sh -c yum -y -q localinstall --nogpgcheck /tmp/zabbix-server-mysql-2.4.0-1.el6.x86_64.rpm] returned a non-zero code: 1

This is a simple workaround forcing the use of centos6