Add debug subcommand that prints applicable configuration

[m90]

In #627 and a lot of other issues, users seem to struggle with setting up their configuration in a way that the tool picks up the expected values. It probably would be helpful (and not too hard) to introduce a debug subcommand that prints a human-friendly version of all configuration values that have been picked up and then exits. Like this, users can pinpoint discrepancies in a way that is easier than guessing / trial and error.

[PythonGermany]

nicholas-fedor/shoutrrr also has a feature like this. Very neat and helpful. Maybe it could be used as inspiration on how to implement such a feature here.

At the moment I'm still struggling to understand why my NOTIFICATION_URLS value works when I pass it directly as env variable but not when I try to load it from a docker secrets file using the env var _FILE postfix. Would be helpful for situations like this and similar to see what config ends up being loaded. Maybe for the NOTIFICATIONS_URLS the shoutrrr verify command could even be integrated into a debug (or verify) command of this project.

[m90]

At the moment I'm still struggling to understand why my NOTIFICATION_URLS value works when I pass it directly as env variable but not when I try to load it from a docker secrets file using the env var _FILE postfix.

Having a trailing newline in Docker secrets has bitten me quite often by now, sounds a bit like this could be your problem too? When creating the secret, it's important to use echo -n or similar.

[PythonGermany]

I've tried the approach of echoing the NOTIFICATION_URLS value I want with echo -n into a file and then it worked. Before I used vim and I did not know it always inserts a line feed character at the end. Thanks for the hint that lead me on the right path. I saw the newline character at the end of the parsed url before in the error logs of the backup container but I did not understand where it was coming from since I never pressed enter in vim and it didn't look like there was a newline after the text.

[m90]

Good to hear it's working and yet another prime example why this issue would be a nice feature.