contrib/apparmor: deny /sys/devices/virtual/powercap

While this is not strictly necessary as the default OCI config masks this
path, it is possible that the user disabled path masking, passed their
own list, or is using a forked (or future) daemon version that has a
modified default config/allows changing the default config.

Add some defense-in-depth by also masking out this problematic hardware
device with the AppArmor LSM.

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

contrib/apparmor/template.go

@@ -77,6 +77,7 @@ profile {{.Name}} flags=(attach_disconnected,mediate_deleted) {
   deny /sys/fs/c[^g]*/** wklx,
   deny /sys/fs/cg[^r]*/** wklx,
   deny /sys/firmware/** rwklx,
+  deny /sys/devices/virtual/powercap/** rwklx,
   deny /sys/kernel/security/** rwklx,
 
   # allow processes within the container to trace each other,