Parallel a-c-r runs interfere in Docker uploads
Copied from https://dev.arvados.org/issues/12355#note-9
If I give cwltest the -j=8 parameter (for instance) to run 8 of these things at a time, arvados-cwl-runner bombs out like this:
2017-10-03 23:13:58 arvados.arv_put INFO: Resuming upload from cache file /root/.cache/arvados/arv-put/c5dadc18a2dc00619c0a24e33ed5e703 2017-10-03 23:13:58 arvados.arv_put ERROR: arv-put: Another process is already uploading this data. Use --no-cache if this is really what you want. 2017-10-03 23:13:58 cwltool ERROR: Workflow error, try again with --debug for more information: v1.0/cat3-tool.cwl:7:5: keepdocker exited with code 1
The failures are all to do with multiple jobs trying to arv-put (the same) docker images via arv-keepdocker.
Need to isolate the arv-keepdocker calls so they either share the work (because they are trying to do the same thing) or at least don't interfere with each other.