Adds rsync to server pipeline docker image.

rsync is a dependency of the management utilities and was causing the gpinitmirror tests in
isolation2 to fail.

concourse/server_pipeline/docker/Dockerfile

@@ -50,7 +50,8 @@ RUN apt update && apt install -y \
     python-psutil \
     python-yaml \
     zlib1g-dev \
-    cpanminus
+    cpanminus \
+    rsync
 
 #
 # Prepare debugging environment