refactor: use new-style docker management commands

This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.

This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.

plugins/apps/post-delete

@@ -16,7 +16,7 @@ app_post_delete() {
   fi
 
   # shellcheck disable=SC2046
-  "$DOCKER_BIN" rmi $("$DOCKER_BIN" images -q "$IMAGE_REPO" | xargs) &>/dev/null || true
+  "$DOCKER_BIN" image rm $("$DOCKER_BIN" image list --quiet "$IMAGE_REPO" | xargs) &>/dev/null || true
 }
 
 app_post_delete "$@"