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

(cherry picked from commit ac4a95b9314cf1f8ce01f42016c271c0a6078372)

live-build/auto/build

@@ -245,7 +245,7 @@ if  [ "$(dpkg-divert --truename /usr/bin/man)" = "/usr/bin/man.REAL" ]; then
 fi
 EOF
 
-                if [ "$PROJECT" != "ubuntu-base" ]; then
+                if [ "$PROJECT" != "ubuntu-base" ] && [ "$PROJECT" != "ubuntu-oci" ]; then
                     	# ubuntu-minimal is too much for a docker container (it contains
                     	# systemd and other things)
 	                cat >> chroot/usr/local/sbin/unminimize <<'EOF'

live-build/auto/config

@@ -515,7 +515,7 @@ if [ "$PREINSTALLED" = "true" ]; then
 		ubuntu-server)
 			add_package live oem-config-debconf ubiquity-frontend-debconf
 			;;
-		ubuntu-core|ubuntu-base|base|ubuntu-cpc)
+		ubuntu-core|ubuntu-base|ubuntu-oci|base|ubuntu-cpc)
 			;;
 		ubuntu)
 			add_package live oem-config-gtk ubiquity-frontend-gtk
@@ -796,6 +796,10 @@ case $PROJECT in
 		OPTS="${OPTS:+$OPTS }--bootstrap-flavour=minimal"
 		;;
 
+	ubuntu-oci)
+		OPTS="${OPTS:+$OPTS }--bootstrap-flavour=minimal"
+		;;
+
 	ubuntu-cpc)
 		KERNEL_FLAVOURS=virtual
 
@@ -957,7 +961,7 @@ case $ARCH in
 esac
 
 case $PROJECT:${SUBPROJECT:-} in
-	ubuntu-server:*|ubuntu-base:*)
+	ubuntu-server:*|ubuntu-base:*|ubuntu-oci:*)
 		OPTS="${OPTS:+$OPTS }--linux-packages=none --initramfs=none"
 		KERNEL_FLAVOURS=none
 		BINARY_REMOVE_LINUX=false
@@ -1024,7 +1028,7 @@ echo "SUBPROJECT=\"${SUBPROJECT:-}\"" >> config/binary
 echo "LB_DISTRIBUTION=\"$SUITE\"" >> config/binary
 
 case $PROJECT in
-  ubuntu-cpc|ubuntu-core|ubuntu-base|base)
+  ubuntu-cpc|ubuntu-core|ubuntu-base|ubuntu-oci|base)
     # ubuntu-cpc gets this added in 025-create-groups.chroot, and we do
     # not want this group in projects that are effectively just chroots
     ;;

live-build/functions

@@ -795,7 +795,7 @@ clean_debian_chroot() {
     rm -f chroot/var/cache/debconf/*-old chroot/var/lib/dpkg/*-old
     Chroot chroot apt clean
     # For the docker images we remove even more stuff.
-    if [ "${PROJECT}:${SUBPROJECT:-}" = "ubuntu-base:minimized" ]; then
+    if [ "${PROJECT}:${SUBPROJECT:-}" = "ubuntu-base:minimized" ] || [ "${PROJECT}:${SUBPROJECT:-}" = "ubuntu-oci:minimized" ]; then
         # Remove apt lists (that are currently removed downstream
         # anyway)
         rm -rf chroot/var/lib/apt/lists/*