feat: motd

The idea here is to use update-motd to pull a simple message from https://eic.github.io/motd, which is shown on interactive logins into the eic-shell container. The message would be dynamic, but a cached version at ~/.eic-news would be shown while a background process pulls a new version. Each news run of a container would not show the latest message, but the next run will. Explictly running eic-news would be equivalent to running motd and showing the latest version as well.