Support release image on modern docker, upgrade guest distro to 22.04

[ConnorNelson]

The first commit fixes #24. It is not clear if #18 is still relevant, but it seems like passing --init is simpler to guarantee our docker-init.

The second commit brings the guest distro to ubuntu 22.04 (Linux 5.15.0-87-generic as of this PR).

[pmorjan]

Thanks for the PR.

The first commit looks good to me.
I think we don't have to support overriding DOCKER_INIT if we instead search for docker-init in $PATH.
Your changes and this should make it work with old and new Docker:

• in initrd/Makefile: remove all references to docker-init
• in initrd/mkinitrd.sh: cp $(command -v docker-init) $TMP_DIR/sbin/

Regarding Ubuntu 22.04
Is there a special reason for switching to 22.04? Keep in mind that we must also support s390x.
I would prefer to stay on 20.04 for now unless there is a good reason to switch.
But we could put an additional Dockerfile.2204 in qemu/x86_64/ together with your change in qemu_amd64.go for people who want to try out 22.04.

[ConnorNelson]

So the issue with searching $PATH for docker-init is that modern installs put it at /usr/libexec/docker/docker-init, which is not in $PATH. We can of course search $PATH:/usr/libexec/docker.

It's not clear to me what the downsides are of --init, but I'm happy to switch the PR to either strategy, just let me know which you prefer and I'll update the PR.

---

As for Ubuntu 22.04, there isn't a specific reason other than getting a newer kernel (I need some newer features). The s390x part of this project is irrelevant to me, but I understand the concern for maintaining support for it. Another option here (though maybe too-clever) would be to parameterize FROM ubuntu:20.04, e.g.:

ARG UBUNTU_VERSION=20.04
FROM ubuntu:$UBUNTU_VERSION

The we have the Makefile pass through an optional --build-arg UBUNTU_VERSION=22.04.

However, the downside, and where we'll introduce some more complexity is that we'll need some system for constructing kernel.conf dependent on our kernel version. For example, in our ubuntu:22.04 version, we needed to add netfs.ko. If you know what are core requirements are, we can dynamically construct this dependency list using lib/modules/*/modules.dep. Or probably, even better, change the logic of init to remove loadKernelModules/kernel.conf and instead use logic more similar in modprobe.go to automatically resolve these dependencies at runtime (probably a tiny performance hit in parsing modules.dep at runtime).

Alternatively, we can add a Dockerfile.2204 and have the Makefile take care of that optionally. However, there may be interest in 24.04 next year 😉.

Again, let me know what strategy you prefer, and I'd be happy to update the PR.

[pmorjan]

I'm sorry for the confusing regarding the PATH. In all my systems the mount path is always /sbin/docker-init. So your commit is ok. But we could consider removing the check in initrd/Makefile. With your changes it's no longer needed.
So please create a PR with your first commit.

[pmorjan]

If /sbin/docker-init does not exist we could simple try to copy from /usr/libexec/docker/docker-init to support "modern installs". You can add this to your PR if you like.

[pmorjan]

Regarding 22.04 I would prefer a second Dockerfile. If we integrate a second distro/version then we must always test and support it. The base kernel modules are copied into the initrd. Therefore we need the kernel.conf. I think we should make it possible to use 22.04+ but I don't want to switch now nor support two versions. There are other things waiting to be fixed, e.g. removing 'pkg/errors'. A 2nd. PR with a Dockerfile.2022 and maybe some instructions in the readme on how to use it is very welcome.

[pmorjan]

Thanks @ConnorNelson
See branch ubuntu2204.