add configure_oci function and use it in ubuntu-oci

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 a81972a58b)

live-build/auto/build

@@ -439,6 +439,9 @@ deb file:/var/lib/preinstalled-pool/ $LB_DISTRIBUTION $LB_PARENT_ARCHIVE_AREAS
 			Chroot chroot "ln -s /etc/media-info /var/log/installer/media-info"
 		fi
 	fi
+	if [ "$PROJECT" = "ubuntu-oci" ]; then
+	    configure_oci chroot
+	fi
 	if [ "$PROJECT" = "ubuntu-cpc" ]; then
 		if [ "${SUBPROJECT:-}" = minimized ]; then
 			BUILD_NAME=minimal

live-build/functions

@@ -608,6 +608,62 @@ snap_preseed() {
     fi
 }
 
+configure_oci() {
+    # configure a chroot to be a OCI/docker container
+    # theses changes are taken from the current Dockerfile modifications done
+    # at https://github.com/tianon/docker-brew-ubuntu-core/blob/master/update.sh
+
+    local chroot=$1
+
+    echo "==== Configuring OCI ===="
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L40-L48
+    echo '#!/bin/sh' > ${chroot}/usr/sbin/policy-rc.d
+    echo 'exit 101' >> ${chroot}/usr/sbin/policy-rc.d
+    Chroot ${chroot} "chmod +x /usr/sbin/policy-rc.d"
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L54-L56
+    Chroot ${chroot} "dpkg-divert --local --rename --add /sbin/initctl"
+    cp -a ${chroot}/usr/sbin/policy-rc.d ${chroot}/sbin/initctl
+    sed -i 's/^exit.*/exit 0/' ${chroot}/sbin/initctl
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L71-L78
+    echo 'force-unsafe-io' > ${chroot}/etc/dpkg/dpkg.cfg.d/docker-apt-speedup
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L85-L105
+    echo 'DPkg::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };' > ${chroot}/etc/apt/apt.conf.d/docker-clean
+
+    echo 'APT::Update::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };' >> ${chroot}/etc/apt/apt.conf.d/docker-clean
+
+    echo 'Dir::Cache::pkgcache ""; Dir::Cache::srcpkgcache "";' >> ${chroot}/etc/apt/apt.conf.d/docker-clean
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L109-L115
+    echo 'Acquire::Languages "none";' > ${chroot}/etc/apt/apt.conf.d/docker-no-languages
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L118-L130
+    echo 'Acquire::GzipIndexes "true"; Acquire::CompressionTypes::Order:: "gz";' > ${chroot}/etc/apt/apt.conf.d/docker-gzip-indexes
+
+    # https://github.com/docker/docker/blob/9a9fc01af8fb5d98b8eec0740716226fadb3735c/contrib/mkimage/debootstrap#L134-L151
+    echo 'Apt::AutoRemove::SuggestsImportant "false";' > ${chroot}/etc/apt/apt.conf.d/docker-autoremove-suggests
+
+    # delete all the apt list files since they're big and get stale quickly
+    rm -rf ${chroot}/var/lib/apt/lists/*
+
+    # verify that the APT lists files do not exist
+    Chroot chroot "apt-get indextargets" > indextargets.out
+    [ ! -s indextargets.out ]
+    rm indextargets.out
+    # (see https://bugs.launchpad.net/cloud-images/+bug/1699913)
+
+    # make systemd-detect-virt return "docker"
+    # See: https://github.com/systemd/systemd/blob/aa0c34279ee40bce2f9681b496922dedbadfca19/src/basic/virt.c#L434
+    mkdir -p ${chroot}/run/systemd
+    echo 'docker' > ${chroot}/run/systemd/container
+
+    rm -rf ${chroot}/var/cache/apt/*.bin
+    echo "==== Configuring OCI done ===="
+}
+
 is_live_layer () {
     local pass=$1
     for livepass in $LIVE_PASSES; do