Dockerfile build fails if XDG_RUNTIME_DIR is not set.

[dtrudg]

Transferring from #3407.... as noted by @nathanweeks

(This is probably an separate issue) It appears that if XDG_RUNTIME_DIR is unset, the singularity-buildkitd unix domain socket is created in /run/singularity-buildkitd/ by default, which would fail in the rootless case (as on a multi-tenant HPC cluster):

$ singularity --debug build --oci test.oci-sif Dockerfile                                                              
DEBUG   [U=1000,P=3154663]persistentPreRun()            Singularity version: 3.7.3+3035-g82588d19a 
...
INFO    [U=0,P=3154678]    ensureBuildkitd()             Did not find usable system buildkitd daemon. Starting built-in singularity-buildkitd.
WARNING [U=0,P=3154678]    startBuildkitd()              ~/.singularity filesystem does not support buildkit. Using temporary directory /tmp/singularity-buildkitd-2111004501. Layers will not be cached for future builds.
DEBUG   [U=0,P=3154678]    startBuildkitd()              starting /home/user/local/singularity/libexec/singularity/bin/singularity-buildkitd [--root=/tmp/singularity-buildkitd-2111004501 --socket=unix:///run/singularity-buildkitd/singularity-buildkitd-3154678.sock]
FATAL:   singularity-buildkitd: permission denied
...

Setting XDG_RUNTIME_DIR to a user-writable path (which may be nonexistent before running singularity build) allows the build to succeed:

$ XDG_RUNTIME_DIR=/tmp/container-user-$UID  singularity build --oci test.oci-sif Dockerfile 
...
INFO:    Did not find usable system buildkitd daemon. Starting built-in singularity-buildkitd.
WARNING: ~/.singularity filesystem does not support buildkit. Using temporary directory /tmp/singularity-buildkitd-4245275241. Layers will not be cached for future builds.
INFO:    singularity-buildkitd: running server on /tmp/container-user-1000/singularity-buildkitd/singularity-buildkitd-3155637.sock
...
WARNING: removing singularity-buildkitd temporary directory /tmp/singularity-buildkitd-4245275241
INFO:    Build complete: test.oci-sif

[dtrudg]

Unfortunately we cannot work around this in a similar way to #3407

Although we can set the singularity-buildkitd socket to a place other than XDG_RUNTIME_DIR, the buildkit code which we use as a dependency relies on XDG_RUNTIME_DIR being set in various places. Cache operations, or invocation of runc during the build will fail.

I don't think we should be setting XDG_RUNTIME_DIR ourselves, to a tmpdir we create. There is a strong assumption that this value is a genuine session specific directory that is configured by systemd, or the user manually.

Will add a better error indicating that XDG_RUNTIME_DIR must be set.