Add portus 2.3 tag

[dforste]

Add 2.3 to the open build service.
https://build.opensuse.org/project/subprojects/Virtualization:containers:Portus
http://port.us.org/docs/how_we_release.html#brief-explanation

[mssola]

I'll do this in due time. For now the branch has not been created yet. We should be doing this in a matter of days 👍