unpack_singularity builds and immediately discards an intermediate SIF

[agitter]

Given the proot problems in #505, we may want to reconsider how we pull and then build sif files

spras/spras/containers.py

Lines 473 to 484 in 07707bf

	# Adding 'docker://' to the container indicates this is a Docker image Singularity must convert
	image_path = Client.pull('docker://' + resolved.image, name=str(unpacked_dir / sif_file))
	base_cont_path = unpacked_dir / Path(base_cont)
	# Check whether the directory for base_cont_path already exists. When running concurrent jobs, it's possible
	# something else has already pulled/unpacked the container.
	# Here, we expand the sif image from `image_path` to a directory indicated by `base_cont_path`
	if not base_cont_path.exists():
	Client.build(recipe=image_path, image=str(base_cont_path), sandbox=True, sudo=False)
	print(f'Resolved singularity image to sandbox: {base_cont_path}', flush=True)
	return base_cont_path # sandbox directory

We could use a single Client.build instead.

The two stage version we have could also introduce a race condition.