Build the XQueue docker image more like the other images

This includes copying some docs and making it more straightforward
for docker-compose to control how we spin it up on devstacks.

docker/build/xqueue/Dockerfile

-FROM edxops/xenial-common:latest
+# To build this Dockerfile:
+#
+# From the root of configuration:
+#
+# docker build -f docker/build/xqueue/Dockerfile . -t edxops/xqueue:latest
+#
+# This allows the dockerfile to update /edx/app/edx_ansible/edx_ansible
+# with the currently checked-out configuration repo.
+
+ARG IMAGE_PREFIX
+FROM edxops/${IMAGE_PREFIX}xenial-common:latest
 MAINTAINER edxops
+CMD ["/edx/app/supervisor/venvs/supervisor/bin/supervisord", "-n", "--configuration", "/edx/app/supervisor/supervisord.conf"]
 
 USER root
 RUN apt-get update
@@ -8,6 +19,4 @@ COPY docker/build/xqueue/ansible_overrides.yml /
 WORKDIR /edx/app/edx_ansible/edx_ansible/docker/plays
 RUN /edx/app/edx_ansible/venvs/edx_ansible/bin/ansible-playbook xqueue.yml -i '127.0.0.1,' -c local -t "install:base,install:system-requirements,install:configuration,install:app-requirements,install:code" -e@/ansible_overrides.yml
 
-COPY docker/build/xqueue/docker-run.sh /
-ENTRYPOINT ["/docker-run.sh"]
 EXPOSE 18040

docker/build/xqueue/docker-run.sh

-#!/bin/bash
-set -e
-
-/usr/sbin/rsyslogd
-/edx/app/supervisor/venvs/supervisor/bin/supervisord --nodaemon --configuration /edx/app/supervisor/supervisord.conf