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>

docker/Dockerfile

@@ -8,11 +8,17 @@ RUN go mod download
 
 COPY . .
 RUN go run build.go
-
 FROM alpine:latest AS restic
-
 RUN apk add --no-cache ca-certificates fuse openssh-client tzdata jq
 
 COPY --from=builder /go/src/github.com/restic/restic/restic /usr/bin
+COPY ./docker/entrypoint.sh /entrypoint.sh
 
-ENTRYPOINT ["/usr/bin/restic"]
+# IO class default is "none"- 0, however busybox reject ionice `-c0 -n<something>`
+# since priority has no meaning for no scheduler.
+# Thus the entrypoint script below is necessary
+ENV IONICE_CLASS=
+ENV IONICE_PRIORITY=4
+ENV NICE=0
+
+ENTRYPOINT ["/entrypoint.sh"]