Experimental support for rootless containers

[Furisto]

This introduces experimental support for rootless containers. To test this:

// create spec for rootless containers
runc spec --rootless

// provide console socket, 
recvtty tty.sock

// in another shell, note the missing sudo
./youki create --bundle <bundlePath> --console-socket <SocketPath>/tty.sock 2309pasf
./youki start 2309pasf

// in container shell
id
uid=0(root) gid=0(root) groups=65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),65534(nobody),0(root)

// on the host, note the user, it is not root
furisto    41875    1562  0 21:48 pts/8    00:00:00  \_ ./youki create --bundle...
furisto    41876   41875  0 21:48 pts/8    00:00:00      \_ /bin/sh

Known limitations for now:

• Resource restrictions will be ignored. This requires systemd support
• Only single id mapping fully supported

[utam0k]

I'll review it later today.

[utam0k]

probably don't need a Result here.

[utam0k]

Since rootless works for now, let's output a warn log that there is an unimplemented feature when this function is true.

[utam0k]

Isn't it also necessary to have uidMapping and gidMapping in the config?

[Furisto]

Yes, this is checked in the validate() function (L50-56)

[utam0k]

@Furisto Is it persistent to switch rootless with environment variables? Or is it temporary?

[Furisto]

I used it during development. I can remove it if you want. So yeah, temporary unless we encounter a use case for which it would be beneficial.

[utam0k]

Suggested change

	for p in paths.split_terminator(':') {
	let binary_path = PathBuf::from(p).join(binary);
	if binary_path.exists() {
	return Ok(Some(binary_path));
	}
	}
	Ok(None)
	Ok(paths
	.split_terminator(':')
	.find(|p| PathBuf::from(p).join(binary).exists())
	.map(PathBuf::from))

[utam0k]

@Furisto
I made a couple of comments, but it worked on my enviroment. It's amazing!
The fork/parent.rs file is too big and may need to be refactored soon, even if it's not this PR. This is an early design mistake on my part.

[Furisto]

@utam0k Addressed your comments

[utam0k]

🎉