Move the VOLUME declaration to the end of the Dockerfile

One a VOLUME has been declared, changes to that location are subsequently dropped during the building of the Dockerfile. With the changes, in rabbitmq/rabbitmq-common#369 applied, this meant that the enabling of plugins no longer worked. We now make the VOLUME declaration the last line of the Dockerfile, assuming that we would not want to throw out any changes.

Additionally, we use gosu to execute plugin enablement, so that the 'enabled_plugins' file is owned by the rabbitmq user when the broker goes to update it.

Author: HoloRin

Dockerfile

@@ -256,8 +256,6 @@ RUN ln -sf /opt/rabbitmq/plugins /plugins
 
 # set home so that any `--user` knows where to put the erlang cookie
 ENV HOME $RABBITMQ_DATA_DIR
-# Hint that the data (a.k.a. home dir) dir should be separate volume
-VOLUME $RABBITMQ_DATA_DIR
 
 # warning: the VM is running with native name encoding of latin1 which may cause Elixir to malfunction as it expects utf8. Please ensure your locale is set to UTF-8 (which can be verified by running "locale" in your shell)
 # Setting all environment variables that control language preferences, behaviour differs - https://www.gnu.org/software/gettext/manual/html_node/The-LANGUAGE-variable.html#The-LANGUAGE-variable
@@ -271,8 +269,8 @@ EXPOSE 4369 5671 5672 25672
 CMD ["rabbitmq-server"]
 
 # rabbitmq_management
-RUN rabbitmq-plugins enable --offline rabbitmq_management && \
-    rabbitmq-plugins is_enabled rabbitmq_management --offline
+RUN gosu rabbitmq rabbitmq-plugins enable --offline rabbitmq_management && \
+    gosu rabbitmq rabbitmq-plugins is_enabled rabbitmq_management --offline
 # extract "rabbitmqadmin" from inside the "rabbitmq_management-X.Y.Z.ez" plugin zipfile
 # see https://github.com/docker-library/rabbitmq/issues/207
 RUN set -eux; \
@@ -296,6 +294,14 @@ RUN set -eux; \
 	rabbitmqadmin --version
 EXPOSE 15671 15672
 
-RUN rabbitmq-plugins enable --offline rabbitmq_prometheus && \
-    rabbitmq-plugins is_enabled rabbitmq_prometheus --offline
+RUN gosu rabbitmq rabbitmq-plugins enable --offline rabbitmq_prometheus && \
+    gosu rabbitmq rabbitmq-plugins is_enabled rabbitmq_prometheus --offline
 EXPOSE 15692
+
+# Hint that the data (a.k.a. home dir) dir should be separate volume
+# We do this last, otherwise our plugin configuration will be thrown out,
+# as per https://docs.docker.com/engine/reference/builder/#volume
+# > Changing the volume from within the Dockerfile:
+# > If any build steps change the data within the volume after it has been
+# > declared, those changes will be discarded.
+VOLUME $RABBITMQ_DATA_DIR