This means that the tree rooted at WORKDIR is on the host filesystem,
and therefore survives system restarts;
otherwise it wouldn't.

This also simplifies file access from containers.
There's no need to copy input files from the slot dir to the container;
they're already in the container.
Similar with output files.
Remove these capabilities from the wrapper.

If a Docker app wants to access files in the project dir
(e.g. if they're huge files, shared with other jobs)
it can do so by mounting the project dir (using job.toml)
and then parsing the link files.

Merge the two test cases (copy and mount) into one
in both standalone and app_test.

Docker chokes on the Windows-style "c:\blah..." path