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>

.dockerignore

@@ -5,6 +5,7 @@
 !/*.go
 !/go.*
 !/cmd/*
+!/docker/entrypoint.sh
 !/internal/*
 !/helpers/*
 !/VERSION