Hello,

first, thanks for this job that saved me a lot of time! I've seen some things that I find a bit strange:

I had a look at the base-mail/Dockerfile, you are adding a lot of new sources but I don't think they are that useful (I cloned your repo and changed the base to debian:stable, and then removed all the sources, (of course,) and I didn't need to change anything else). Actually, I don't find the mail-base image that useful...

Also, some things like owncloud/public_url or mail-base/domains could just be environment variables, that would be more flexible than a small plain text file added at build time.

Last, you are using make to build the images, why not using docker-compose instead ?

I'm not that familiar with mail servers, but when I'll get a decent internet connection (may the 13th, normally) I'll try to improve these and make a pull request, if you are interested in.