It is a short, concicse name for the compose (and one of the ways that docker officially recommends to name the files), and jinja is the careful recommendation of the jinja docs. https://jinja.palletsprojects.com/en/stable/templates/#template-file-extension
Adapted from cloudserve-infrastructure, implements a backup stack using restic. The actual backups have to be implemented by individual other roles but this sets up initialization, pruning and checking of a repository.