osmo-uecups-latest: install missing iputils-ping

ping is needed for ttcn3-pgw-test.  In the case of osmo-uecups-master
it's available from the parent $DISTRO-build image, but for the -latest
it needs to be installed by the inherited image itself.

Change-Id: Ia1b9124f09203c7dcf6d26dee78b09d5a139a04f
Related: OS#5913

osmo-uecups-latest/Dockerfile

@@ -9,6 +9,7 @@ RUN	case "$DISTRO" in \
 	debian*) \
 		apt-get update && \
 		apt-get install -y --no-install-recommends \
+			iputils-ping \
 			osmo-uecups && \
 		apt-get clean \
 		;; \