Exim4 not installing

I used this docker and successfully see the mailman web interface.

However if it doesn't send and receive mail.  If I get a shell inside the docker exim4 is not there.  If I run "apt-get update" and then "./run.sh" then I get exim4 (until the next time someone refreshes the docker container of course).

I don't have much docker experience but I'll see if I can fork this and make a patch.