Make sure check for presence of maintainer check works

[TomasTomecek]

We've got a report internally that $subject does not work. Please investigate and ideally create a test case for this.

I am pretty sure the "WARN:Label 'maintainer' has to be specified." is a
false warning, unless I am blind and can not see the typo in my Dockerfile.

CC @fkluknav

Edit: I think I get it know, the message is wrong. We check whether the content matches a certain regex.

[fkluknav]

LABEL maintainer="Red Hat, Inc." causes WARN:Label 'maintainer' has to be specified.

[TomasTomecek]

Yeah, our regex is @redhat.com$. This is worth discussing internally.

[fkluknav]

What is the standard for 'maintainer' label in images from Red Hat? Is there any? A person? A group? A company?

[TomasTomecek]

Exactly, probably there is none. Therefore this should be brought up internally so we all agree on one standard.

@fkluknav Would you like to bring it up or should we take care of it?