contrib/docker/centos6

docker: use stable tags for fedora and centos

A moving target is rarely useful.

FROM centos:centos6
RUN yum install -y gcc
RUN yum install -y python-devel python-docutils
RUN yum install -y make
RUN yum install -y rpm-build
RUN yum install -y gettext
RUN yum install -y tar