Image version pinning

[luxaritas]

Followup to the changes merged in #44 - is there a reason why the version was not changed to be pinned in the main docker-compose.yml? By defaulting to "latest", it is now possible to have a commit checked out that doesn't work because the rest of the configuration is not in sync with the image that gets run - either I have an old commit checked out and the container that gets loaded is newer than it should be, or I pin the container version in the override myself, pull, forget to update, and now the image is older than it should be.