feat: add UBI image support

[Victoremepunto]

based on the spirit of #5 and since I thought it may be really useful to have UBI based images for caddy.

[francislavoie]

I think this needs a Dockerfile.ubi.tmpl similar to https://github.com/caddyserver/caddy-docker/blob/master/Dockerfile.tmpl so that the codegen produces the files correctly.

[Victoremepunto]

I think this needs a Dockerfile.ubi.tmpl similar to https://github.com/caddyserver/caddy-docker/blob/master/Dockerfile.tmpl so that the codegen produces the files correctly.

Thanks @francislavoie , I just submitted it!

[Victoremepunto]

@francislavoie ping! Hello , what is required for this to move forward ? do I need to do anything else ?

Also, would it be better to set it to UBI-8 instead (more stable) ?

I also saw #369 and thought it might be really helpful indeed, esp. for running in Openshift clusters, I can incorporate those changes in this PR as well.