Update Troubleshooting section in using_docker_build.md

doc/ci/docker/using_docker_build.md

@@ -728,14 +728,18 @@ is set to `always`.
 [2fa]: ../../user/profile/account/two_factor_authentication.md
 [pat]: ../../user/profile/personal_access_tokens.md
 
-<!-- ## 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.
+### docker: Cannot connect to the Docker daemon at tcp://docker:2375. Is the docker daemon running?
 
-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. -->
+This is a common error when you are using
+[Docker in Docker](#use-docker-in-docker-workflow-with-docker-executor)
+v19.03 or higher.
+
+This occurs because Docker starts on TLS automatically, so you need to do some set up.
+If:
+
+- This is the first time setting it up, carefully read
+  [using Docker in Docker workflow](#use-docker-in-docker-workflow-with-docker-executor).
+- You are upgrading from v18.09 or earlier, read our
+  [upgrade guide](https://about.gitlab.com/blog/2019/07/31/docker-in-docker-with-docker-19-dot-03/).