Service hostname also include registry name, when using non Docker Hub registry.

What does this MR do?

Updated documentation

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

Check for typos:P

Why was this MR needed?

I came across this issue today and spent some time resolving it. Having it documented could save some time

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

None