Add information on setting up Docker executor so that it can talk to host network

This is a general Merge Request template. Consider to choose a template from the list above if it will match your case more.

What does this MR do?

Extends the documentation to cover a tricky case with Docker and GitLab being installed on the same machine and Docker executor needing to be configured before being ready to go.

Why was this MR needed?

Because the existing documentation is not accurate. The Docker executor is not ready without extra configuration whereas the documentation claims it will be right after registering it.

The error messaging is too generic and online resources do not provide enough guidance for non-pros to figure it out easily.

Are there points in the code the reviewer needs to double check?

Reviewers should suggest better wording, check English and suggest better structure or placement of the change within the file.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?