misc: Add --all to Runner docker system prune

Without `--all` `docker prune --force --volumes` will remove everything
exception non-dangling images. For an image to be considered dangling it
must be untagged and/or not used by a container at that time. As most of
the images we download are tagged (e.g., `:latest`) then most of our
images are never removed without the inclusion of `--all` which will
remove any image not currently used by a container.

Images were starting to accumulate on runners. This will ensure they do
not and are cleaned after each job run.

Change-Id: I6d8441a11d22fdcf827e9c44422dbcf02cf600e0

util/github-runners-vagrant/action-run.sh

@@ -77,5 +77,5 @@ while true; do
 
     # 4. Cleanup the machine
     rm -rf "${WORK_DIR}"
-    docker system prune --force --volumes
+    docker system prune --force --volumes --all
 done