feat: add integrated nice and ionice options for docker (#5448)

The intended usage here is to basically kick restic as a background
"do it, but don't bother my normal load" process.

This allows passing the following environment variables in to
influence scheduling:

- NICE: usual CPU nice.  Defaults to 0.  This requires CAP_SYS_NICE
  to set a negative nice (IE, prioritize).
- IONICE_CLASS: usual ionice class.  Note that setting realtime
  requires CAP_SYS_ADMIN.  Also note the actual ionice default
  is "none".
- IONICE_PRIORITY: set the priority within the given class.  Ignored
  if no class is specified due to class default of "no scheduler".

---------

Signed-off-by: Brian Harring <ferringb@gmail.com>
Co-authored-by: Michael Eischer <michael.eischer@fau.de>

changelog/unreleased/pull-5448

@@ -0,0 +1,11 @@
+Enhancement: Allow nice and ionice configuration for restic containers
+
+The official restic docker now supports the following environment variables:
+
+`NICE`: set the desired nice scheduling.  See `man nice`.
+`IONICE_CLASS`: set the desired I/O scheduling class.  See `man ionice`.  Note that real time support requires the invoker to manually add the `SYS_NICE` capability.
+`IONICE_PRIORITY`: set the prioritization for ionice in the given `IONICE_CLASS`.  This does nothing without `IONICE_CLASS`, but defaults to `4` (no priority, no penalties).
+
+See https://restic.readthedocs.io/en/stable/020_installation.html#docker-container for further details.
+
+https://github.com/restic/restic/pull/5448