Add a command to trigger entrypoint cache creation

stevedore will cache the entrypoint scan when needed. Since we
just installed the things here, do an openstack --help to cause
the entrypoints to get scanned at build time and for the cache
file to be written into the container image.

Change-Id: I73502be6d68c4a38561c9524b4def3c6a6f61ac6

Dockerfile

@@ -23,4 +23,7 @@ FROM docker.io/opendevorg/python-base:3.7
 COPY --from=builder /output/ /output
 RUN /output/install-from-bindep
 
+# Trigger entrypoint loading to trigger stevedore entrypoint caching
+RUN openstack --help >/dev/null 2>&1
+
 CMD ["/usr/local/bin/openstack"]