Add sethomevar hook

[matteo-chesi]

In order to replace container HOME environment entry with HOME value for the running user from the system (getent passwd)
It doesn't use HOME variable from user environment, but from the system.

Keep in mind, there is nothing that will make the user land in HOME.

[fcruzcscs]

I see that when reading the .process.user.uid if this does not exist it will create it to be 0, i.e. root

This makes sense from podman point of view but I am not sure this is something that the user expects, and it is not easy to debug (given that the intent is to give access to the user's home)

maybe is best to error out? worst case, the user would be aware and can disable the hook?

Also, under most conditions skybox should set keep-id, so uid should be correctly populated, thus not being there might mean something else is going wrong

[Madeeks]

I see that when reading the .process.user.uid if this does not exist it will create it to be 0, i.e. root

This makes sense from podman point of view but I am not sure this is something that the user expects, and it is not easy to debug (given that the intent is to give access to the user's home)

maybe is best to error out? worst case, the user would be aware and can disable the hook?

Also, under most conditions skybox should set keep-id, so uid should be correctly populated, thus not being there might mean something else is going wrong

The OCI runtime spec states that uid is a required property on POSIX systems: https://github.com/opencontainers/runtime-spec/blob/main/config.md#posix-platform-user

Therefore, the only circumstance where I imagine this failing is when process does not exist, which OCI specs allow only when the start lifecycle action is not called (e.g. the container is just being created but not started yet).
Normally, sarusctl and Skybox should not fall into this case, because they both invoke a podman run command directly.

However, I'm also thinking that this hook is intended for the specific use case of Skybox, where containers run with keep-id and the host env is propagated in the container, hence the need to insure that HOME is actually usable in case some code wants to use it.
Hence, given the specific scope of the use case, returning an error is probably not a bad idea.