Improvements for build nodes documentation

[bedroge]

• When using something else than /tmp for $EESSI_TMPDIR, you may have to bind mount that location by passing -B $EESSI_TMPDIR to Singularity. We can probably just always use this flag in the instructions, even when /tmp is being used.
• Maybe have a TLDR section that allows you to easily copy all commands in one go.
• Add an extra warning about the working directory from which the tarball gets created; this really has to be a subdir of the repo, as illustrated in the example.
• Add a section on how this can be run as a (non-interactive) job. (using startprefix <<< buildscript.sh)
• Build nodes should have internet access
• How to set the number of cores to be used for compiling
• Easybuild should use a temporary dir in $EESSI_TMPDIR (which can be set to something else than /tmp), or we should bind mount a subdir of $EESSI_TMPDIR as /tmp in the container
• Check available disk space (~30GB?)
• Make the tarball (especially when you install some additional packages, but it can be done in any case) in ${EESSI_TMPDIR}/overlay-upper/${EESSI_PILOT_VERSION}/software/linux