CORD-762 use entrypoint instead of cmd

Change-Id: Icdfd0283e6d22147261f1f11d6facc95b29f4717
diff --git a/containers/xos/Dockerfile.client b/containers/xos/Dockerfile.client
index 2a60577..96e8ca6 100644
--- a/containers/xos/Dockerfile.client
+++ b/containers/xos/Dockerfile.client
@@ -22,4 +22,4 @@
 # install the client library and xossh
 RUN chdir /tmp/xos_client && make
 
-CMD xossh
+ENTRYPOINT ["xossh"]