docker-registry/Dockerfile.template: Adjust Python 2 dependency

Now that the ancestor gentoo-python image is removing Python 2, we
need to get it back for docker-registry.  We need to keep the
USE_PYTHON setting (with a tweaked value) because Gunicorn and
Python-RSA haven't transitioned to the *-r1 Python eclasses yet:

  docker run --rm wking/docker-registry /bin/bash -c 'grep inherit /var/db/pkg/  */*/*ebuild' | grep distutils | grep -v -- '-r1'
  /var/db/pkg/dev-python/rsa-3.1.1/rsa-3.1.1.ebuild:inherit distutils
  /var/db/pkg/www-servers/gunicorn-0.17.4/gunicorn-0.17.4.ebuild:inherit distutils eutils

docker-registry/Dockerfile.template

@@ -26,9 +26,11 @@ FROM ${NAMESPACE}/gentoo-syslog:${TAG}
 MAINTAINER ${MAINTAINER}
 #VOLUME ["${PORTAGE}:/usr/portage:ro", "${PORTAGE}/distfiles:/usr/portage/distfiles:rw"]
 RUN echo 'USE="${USE} sqlite"' >> /etc/portage/make.conf
-RUN echo 'PYTHON_TARGETS="python2_7"' >> /etc/portage/make.conf
-RUN echo 'PYTHON_SINGLE_TARGET="python2_7"' >> /etc/portage/make.conf
+RUN sed -i 's/\(PYTHON_TARGETS\)=.*/\1="python2_7"/' /etc/portage/make.conf
+RUN sed -i 's/\(PYTHON_SINGLE_TARGET\)=.*/\1="python2_7"/' /etc/portage/make.conf
 RUN echo 'USE_PYTHON="2.7"' >> /etc/portage/make.conf
+RUN emerge -v --newuse --deep --with-bdeps=y @system @world
+RUN eselect python set $(eselect python show --python2)
 ADD package.accept_keywords /etc/portage/package.accept_keywords
 RUN emerge -v dev-python/blinker dev-python/boto dev-python/backports-lzma dev-python/flask dev-python/gevent dev-python/pyyaml dev-python/redis-py dev-python/requests dev-python/rsa dev-python/simplejson dev-python/sqlalchemy dev-vcs/git www-servers/gunicorn
 RUN eselect news read new