Properly handle non-deployed applications during apps:rename

An application which has not been deployed will not have an image
available, and therefore attempting to remove an (empty) cache directory
will fail. Rather than checking for the image - which we should
still do, as not having an image for a deployed app is an error state -
we should instead only attempt the docker cache removal if the directory
cannot be removed via rmdir.

Closes #2294

plugins/apps/subcommands/rename

@@ -13,7 +13,9 @@ apps_rename_cmd() {
   local NEW_APP="$3"
 
   mkdir -p "$DOKKU_ROOT/$NEW_APP"
-  docker run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$DOKKU_ROOT/$OLD_APP/cache:/cache" "dokku/$OLD_APP" chmod 777 -R /cache
+  if ! rmdir "$DOKKU_ROOT/$OLD_APP/cache" > /dev/null 2>&1; then
+    docker run "$DOKKU_GLOBAL_RUN_ARGS" --rm -v "$DOKKU_ROOT/$OLD_APP/cache:/cache" "dokku/$OLD_APP" chmod 777 -R /cache
+  fi
   rm -rf "$DOKKU_ROOT/$OLD_APP/cache"
   cp -a "$DOKKU_ROOT/$OLD_APP/." "$DOKKU_ROOT/$NEW_APP"
   DOKKU_APPS_FORCE_DELETE=1 apps_destroy "$OLD_APP"

tests/unit/10_apps.bats

@@ -68,4 +68,17 @@ load test_helper
   echo "output: "$output
   echo "status: "$status
   assert_success
+
+  run dokku apps:create $TEST_APP
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+  run bash -c "dokku apps:rename $TEST_APP great-test-name"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output $TEST_APP
+  run bash -c "dokku --force apps:destroy great-test-name"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
 }