[Merge] ~toabctl/livecd-rootfs:new-ubuntu-oci into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

It was pointed out to me that the tarballs at http://cdimage.ubuntu.com/ubuntu-base/ are also used for hardware bring up and bootstrapping in addition to the core snaps, so we they should not be built with docker customizations (I wonder if for that purpose, building them with SUBPROJECT=minimized really makes sense but well, different topic) but this doesn't change my overall conclusion that this should be merged as is, so I'll do that now.
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https://code.launchpad.net/~toabctl/livecd-rootfs/+git/livecd-rootfs-1/+merge/401596
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