Description of the Pull Request (PR):

Work in progress....

Use a TMPDIR location if the ~/.singularity/singularity-buildkitd location is on a filesystem that won't work for buildkit (as determined by overlay.CheckUpper which is used elsewhere also).

This should fix #3382 but the current PR is not suitable to merge. Exit handling is pretty nasty due to the flow in which singularity-builtkitd is started, and waited for, by singularity as a buildkit client. We don't really have a nice termination route prior to the GRPC server being started... but the TMPDIR and config needs to be setup before that.

Needs re-work of how we launch singularity-buildkitd and catch errors client-side that are earlier / not visible across the client->server socket.

This fixes or addresses the following GitHub issues:

• Fixes singularity build --oci fails when $HOME/.singularity/singularity-buildkitd is on NFS #3382

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