Semaphore images update

[Allison Karlitskaya]

[tl;dr: rebase all PRs on master, tell lis about any problems with semaphore.]

hi all,

I've updated the semaphore images today to a Ubuntu 19.04, as a test.
You'll need to ensure that any pull requests that you submit are
rebased on top of the commits from
https://github.com/cockpit-project/cockpit/pull/12255 in order for
your builds to succeed.  These commits are on master since last night.

I'll be trying to keep an eye on things over the course of the day,
but if you see anything suspicious, please let me know.

Thanks, and happy hacking!

Allison