Change default user that Amlen runs as.

[jonquark]

It's possible to choose what user Amlen runs as, as described in the Dockerfiles: https://github.com/eclipse/amlen/blob/main/server_build/docker_build/Dockerfile.imaserver

But by default if no user is specified - it runs as root. This is bad practice, under this issue we'll:

[ ] Change it to user user amlen, group amlen by default (and create them if they don't exist) [ ] Use files with amlen rather that messagesight in the filename for usernames (but fail back to messagesight ones if they exist and the amlen ones don't) [ ] Document better the ways to set the user (including the amlen->messagesight fallback)