livecd-rootfs

With that, the Dockerfile modifications[0] currently done externally
are done now here. That means that the created rootfs tarball can be
directly used within a Dockerfile to create a container from scratch:

FROM scratch
ADD livecd.ubuntu-oci.rootfs.tar.gz /
CMD ["/bin/bash"]

[0]
https://github.com/tianon/docker-brew-ubuntu-core/blob/master/update.sh

(cherry picked from commit a81972a58b004897bf3e5c14ff371bc2f6b5e4b8)

3ad6996... by Thomas Bechtold on 2021-04-20

Add new ubuntu-oci project

This is a copy of the ubuntu-base project.
Currently ubuntu-base is used as a base for the docker/OCI container
images. The rootfs tarball that is created with ubuntu-base is
published under [0]. That tarball is used in the FROM statement of the
Dockerfile as base and then a couple of modifications are done inside
of the Dockerfile[1].
The ubuntu-oci project will include the changes that are currently
done in the Dockerfile. With that:

1) a Dockerfile using that tarball will be just a 2 line thing:

   FROM scratch
   ADD ubuntu-hirsute-core-cloudimg-amd64-root.tar.gz /
   CMD ["/bin/bash"]

2) Ubuntu has the full control about the build process of the
docker/OCI container. No external sources (like [1]) need to be
modified anymore.
3) Ubuntu can publish containers without depending on the official
dockerhub containers[2]. Currently the containers for the AWS ECR
registry[3] use as a base[4] the official dockerhub containers. That's
no longer needed because a container just needs a Dockerfile described
in 1)

When the ubuntu-oci project has the modifications from [1] included,
we'll also update [1] to use the ubuntu-oci rootfs tarball as a base
and drop the modifications done at [1].

Note: Creating a new ubuntu-oci project instead of using ubuntu-base
will make sure that we don't break users who are currently using
ubuntu-base rootfs tarballs for doing their own thing.

[0] https://partner-images.canonical.com/core/
[1]
https://github.com/tianon/docker-brew-ubuntu-core/blob/master/update.sh
[2] https://hub.docker.com/_/ubuntu
[3] https://gallery.ecr.aws/ubuntu/ubuntu
[4]
https://launchpad.net/~ubuntu-docker-images/ubuntu-docker-images/+oci/ubuntu/+recipe/ubuntu-20.04