Let's have UBI based images, part 2

[Victoremepunto]

This is a cont. of #5

We use Caddy in a few places on some of the projects I work on, and since we actually build a custom image with caddy based on UBI, I thought it may be useful to reopen the discussion.

I've created #372 to try and close this issue, hope it makes sense.

[francislavoie]

Apparently since we last discussed it, it's now allowed for Docker Official Images docker-library/official-images#9445