This uses the opendev assets bundle image created with
I3166679bde6d771276289b9d32e7e4407957b2f8.
The mount options require using BuildKit, hence the Dockerfile update.
Otherwise conceptually it's fairly simple; copy in the files from the
opendevorg/assets image rather than the file-system.
Change-Id: I36bdc76471eec5380a676ebcdd885a88d3985976