contrib/docker/centos6

gpg: use 'wvfs' instead of 'wfile'

Method is about to be deprecated and the modern form is shorter.

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
# For creating repo meta data
RUN yum install -y createrepo