Add tests for Docker config read when no username is specified

What does this MR do?

Add tests for Docker config read.

Why was this MR needed?

There are no tests for this functionality.

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

I do not think so.

Does this MR meet the acceptance criteria?

• [+] Documentation created/updated
• [+] Added tests for this feature/bug
• [+] In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Issue was not open for this change.