validation: sidekick aci should remove tmp file in post-run (or make a note)

[markllama]

When the ace-validator-sidekick image is run per the instructions on the Rocket "App Container Basics" https://github.com/coreos/rocket#app-container-basics it will leave a file /tmp/sidekick. A subsequent run will fail noting "/db/sidekick unexpectedly exists".

Either the ace-validator-sidekick aci post run should remove the file so that subsequent runs will pass
or it should make a note to STDOUT that the file must be removed before a follow up run will pass.

[jonboulle]

Actually the way I want to solve this is for the requirement to be that the mount point is an empty volume; hence, keeping the checks in the ace validator is fine. Does that make sense? Any chance you could put together a PR to explain this?

[jonboulle]

Filed rkt/rkt#378 which is how I'd expect this to be fixed in rocket