docker snap commands fail to run from a tempdir

While developing the docker snap tests, all docker commands failed if I ran them from checkbox jobs but succeeded from the command line.

After some debugging, the problem comes from the plainbox controller (UserJobExecutionController) where all commands are started from a specific cwd set as a temp dir.

It seems that docker commands (at least the one from the snap) don't really like this method:

--------------[ Running job 4 / 8. Estimated time left: unknown ]---------------
---------------[ Download and run trivial hello-world container ]---------------
ID: 2013.com.canonical.certification::docker/hello-world
Category: 2013.com.canonical.certification::docker
... 8< -------------------------------------------------------------------------
unable to change to original directory. errmsg: No such file or directory
------------------------------------------------------------------------- >8 ---

Since the execution problem comes from plainbox, I'd first fix our controller before submitting a bug for docker.

My proposal for a fix would be a new flag that would prevent running job command from a tempdir cwd (I tested this approach successfully)