Use older version of CentOS for Debian wheezy compatibility

CentOS 7 uses a version of glibc that is not available in Wheezy.
CentOS 7 uses GLIBC_2.14, Debian Wheezy has GLIBC_2.13

Dockerfile

-FROM centos:centos7
+FROM centos:centos6
 MAINTAINER Jørn Åne de Jong <jorn.dejong@uninett.no>
 RUN yum -y install tar make ruby-devel rubygems gcc rpm-build autoconf
 RUN gem install fpm