Enhancement: Configure nice and ionice in the official Docker image

The container entrypoint now reads optional scheduling hints from the
environment:

The environment variable `NICE` sets the process nice value (see `man nice`).

The environment variable `IONICE_CLASS` selects the I/O scheduling class (see
`man ionice`). Real-time classes need the `SYS_NICE` capability added to the
container.

The environment variable `IONICE_PRIORITY` sets the priority within
`IONICE_CLASS` and has no effect unless `IONICE_CLASS` is set; it defaults to `4`
(neutral priority).

Further detail:
https://restic.readthedocs.io/en/stable/020_installation.html#docker-container

https://github.com/restic/restic/pull/5448
