Add 403 troubleshooting steps

Provide the information that 403 errors might
originate from outside proxies

doc/ci/docker/using_kaniko.md

@@ -106,14 +106,10 @@ Guided Exploration project pipeline. It was tested on:
 The example can be copied to your own group or instance for testing. More details
 on what other GitLab CI patterns are demonstrated are available at the project page.
 
-<!-- ## Troubleshooting
+## Troubleshooting
 
-Include any troubleshooting steps that you can foresee. If you know beforehand what issues
-one might have when setting this up, or when something is changed, or on upgrading, it's
-important to describe those, too. Think of things that may go wrong and include them here.
-This is important to minimize requests for support, and to avoid doc comments with
-questions that you know someone might ask.
+### 403 error: "error checking push permissions"
 
-Each scenario can be a third-level heading, e.g. `### Getting error message X`.
-If you have none to add when creating a doc, leave this section in place
-but commented out to help encourage others to add to it in the future. -->
+If you receive this error, it might be due to an outside proxy. Setting the `http_proxy`
+and `https_proxy` [environment variables](../../administration/packages/container_registry.md#running-the-docker-daemon-with-a-proxy)
+can fix the problem.