Non-root docker user, docker compose and health ping

[maxfrei750]

Hi Helmuth! Thanks for the nice work in this repo.

I used it to implement a customized solution to create remote backups of MediaWiki instances, based on rsnapshot: https://github.com/maxfrei750/MediaWikiBackup

While doing so, I implemented a few features that may (or may not 😅) be interesting to you:

• Running as a non-root docker user, which was surprisingly cumbersome, because of cron. I read in local backup with another user as root? #8 that this might mess with permissions when creating a local backup. For my application that's no problem, because it always runs remote, but I think we could work around this limitation, by mapping the user into the container using docker compose.
• Using docker compose. I like it, because I can set all the settings in the docker-compose.yaml and it makes starting the container a breeze.
• I added a way to send a ping to a health check site like healthchecks.io (I'm not affiliated with them). IMHO, very helpful for backups. This could be extended to allow users to make use of the pre- and post- hook of rsnapshot. Although I think that should be possible with your code already.

Feel free to take anything that you deem helpful from my code and of course, I'm happy to answer questions, if necessary. If none of the things are of interest, then just close this issue. 😉