Add backup CLI command

[sissbruecker]

Adds a CLI command that allows to create a backup copy of the SQLite database within the Docker container. The file can then be moved to the host system using docker cp.

Closes #454

[fs111]

I don't quite understand this feature. Are you not mounting a host directory inside the container to keep the db safe? I am currently simply copying the file in a cron job and that just works. Am I missing something?

[sissbruecker]

Technically the copy approach is not safe: https://www.sqlite.org/howtocorrupt.html

Thus an explicit command for creating a backup.

[fs111]

fair enough. My little linkding has so little traffic that I assume it is safe to do. There is no active transaction and if the file is corrupt I get a fresh one 4 hours later.

[sissbruecker]

Yes, I would also assume there is an extremely low risk for the linkding use-case. But for an official recommendation for backups I'd rather go the safe route.