Allow uses to define the path for the urunc configuraton

[cmainas]

Currently the path for the urunc configuration is hardcoded (see https://github.com/urunc-dev/urunc/blob/main/pkg/unikontainers/urunc_config.go#L25) and it expects it to be at /etc/urunc/config.toml. There is no reason to keep the configuration path hardcoded. Instead, we should allow uses to define its path.

Ultimately, the handling of configuration must transfer from urunc to the containerd shim. However, as a temporary solution we can use an environment variable to define the path and have as a fallback /etc/urunc/config.toml.

[Anand-240]

Thanks for splitting this out @cmainas. The env var with a fallback to /etc/urunc/config.toml sounds right for a temporary solution, agreed it isn't worth a heavier resolver if config handling moves to the shim later.
I'll implement it as URUNC_CONFIG_PATH if set, otherwise the current /etc/urunc/config.toml. One thing to keep in mind is that the config is loaded both in the shim and in the urunc binary (guest_rootfs.go on the shim side, create.go and main.go on the runtime side), so the env var has to reach both processes. I'll call that out in the docs.
Happy to take this one, could you assign it to me? I'll add unit tests and a short note in docs/configuration.md.

[cmainas]

Hello @Anand-240 ,

If the environment variable is set in the shim, then it will reach urunc too.

[Anand-240]

Ah got it, thanks. That's simpler then, since urunc inherits the shim's env just setting URUNC_CONFIG_PATH in the shim covers both. I'll get started and open a PR.

[Anand-240]

@cmainas I've put up the PR for this. I kept it as a draft for now since I'm at the 2 open PR limit and drafts don't count against it. Happy to mark it ready as soon as one of my other PRs (#860 or #868) gets reviewed, or let me know how you'd prefer to proceed.