Fix --registry option (bsc#1226436)

[nadvornik]

What does this PR change?

This PR fixes merging of a container image name with a registry specified separately.

Test coverage

• No tests: manual testing

• DONE

Links

https://bugzilla.suse.com/show_bug.cgi?id=1226436

• DONE

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[mbussolotto]

I'm not sure what can happen if Registry is empty (I think this is not the case for suma but it should be for uyuni).
If everything for suma and uyuni works fine without provide --registry (or pass registry in /etc/uyuni/uyuni-tools ) is ok for me

[nadvornik]

I have adjusted the unit test - IMO it is not correct to expect that the path in the alternative registry is the same (see the bug)

Empty registry works OK - the condition !strings.Contains(imageFlags.Name, imageFlags.Registry) is not triggered.

[mbussolotto]

LGTM