Create bind destinations during build

[dtrudg]

When a -B / --bind option is used with singularity build, the bind mount target must be present in the image before the %post step executes.

This makes it a little awkward to use -B with singularity build, as generally a well-known existing path must be used (e.g. /mnt), or directories must be created in the rootfs via a %setup block.

It's not entirely straightforward to implement this sensibly.

• Needs to work across all runtime modes... setuid, kernel overlay / fuse overlay, etc.
• Should the directory be removed after %post if it didn't exist before %post.
• What happens if we bind a directory, but it is also created in %post... or similar?