Add variable for additional backup directories

It would be great to have an environment variable that adds different directories to the backup, so I can back up other directories besides the `/data` directory.

For example, I want to back up my patches in the `/patches` directory and `/data`, which I don't think is currently easy to do.

Start by locating how the container currently selects the /data directory and reads environment variables. Trace the backup entry point and identify the existing checks around included paths. Done means an environment variable can add paths such as /patches while retaining /data in the backup, with the behavior verified using the repository's available checks.

Written by the indexing model from the issue text.