build container hostname does not conform to rfc1035

What does this MR do?

Ensures the hostname of the build container name conforms to DNS RFCs.

Why was this MR needed?

When sending e.g. e-mails the name of the sending server is set automatically from the hostname. Some receiving parties check the validity of the sending hostname.

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

Do not know :-)

Does this MR meet the acceptance criteria?

• Documentation created/updated - I do not think this needs documentation as it is a bugfix
• Added tests for this feature/bug - testing this with mocks is useless, as the flow does not change and only old, 20 character GitLab-CI runner tokens contain invalid characters
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #4779 (closed)