Update detection to check specifically for installation status not ju…

…st presence in the database.

This was providing a false positive if the package was in the apt-cache but in 'uninstalled' state, and on the other side a false negative if the package wasn't in the cache.

src/rocker/templates/rmw_snippet.Dockerfile.em

@@ -4,7 +4,7 @@
 @[ if rmw ]@
 RUN \
   if [ -z "${ROS_DISTRO}" ]; then echo "ROS_DISTRO is unset cannot override RMW" ; exit 1 ; fi ;\
-  if dpkg -l @(' '.join(packages)) > /dev/null 2>&1; then \
+  if ! dpkg -l @(' '.join(packages)) | grep -q ^ii ; then \
    apt-get update \
    && DEBIAN_FRONTENT=non-interactive apt-get install -qy --no-install-recommends\
      @(' '.join(packages)) \