Get rid of overriding convention for environment variables.

[nigelgbanks]

See this here: https://github.com/Islandora-Devops/isle-buildkit/blob/3ae6111ee6fb942e4233154f09365f016356ac97/nginx/README.md#settings

It's done in a bunch of places and is generally confusing, I think we should just bin it.

[joecorall]

At the tech call there was was some questions about what this issue intends to do. IIUC what's being proposed is e.g. in the mariadb image instead of overriding DB_ROOT_PASSWORD env var from the base image as MARIADB_ROOT_PASSWORD we can just have mariadb use DB_ROOT_PASSWORD

[nigelgbanks]

If I recall correctly, there were cases (in particular when we used to support Matomo) in which some users had more than one database service. Also, for some, passwords were managed in a central service like etcd or some other use case. So there was a need to distinguish multiple passwords in the early days, depending on the backend used. That being said, I'm pretty sure no-one is using etcd these days and I think the use case is more or less dead.