refactor to explicitly set IMAGE via get_app_image_name()

plugins/enter/commands

@@ -4,8 +4,8 @@ source "$(dirname "$0")/../common/functions"
 
 case "$1" in
   enter)
-    APP="$2"; TAG=$(get_running_image_tag $APP); CONTAINER_TYPE="$3"
-    verify_app_name "$APP" "$TAG"
+    APP="$2"; CONTAINER_TYPE="$3"; IMAGE_TAG=$(get_running_image_tag $APP); IMAGE=$(get_app_image_name $APP $IMAGE_TAG)
+    verify_app_name "$APP"
 
     [[ ! -n "$3" ]] && dokku_log_fail "No container id specified"