docker-compose files should not use `latest` version on Docker images (at least on tags)

Hi,

as per title, docker-compose files should not use `latest` version on Docker images, at least on tags/releases. As it is right now, f.ex., latest mordred is downloaded for any given release, which means unstable code could be unadvertedly put on production.

(other than that, great project, kudos on all your hard work!)