Add --detach -d option

[adrianreber]

To implement checkpoint/restore containers need to be started with --detach. This is currently missing.

The important part for checkpoint/restore is that --detach does setsid(). Hardcoding setsid() already helped me to be able to checkpoint a container.

diff --git a/crates/libcontainer/src/process/container_intermediate_process.rs b/crates/libcontainer/src/process/container_intermediate_process.rs
index b2a5e0d..a9e1f00 100644
--- a/crates/libcontainer/src/process/container_intermediate_process.rs
+++ b/crates/libcontainer/src/process/container_intermediate_process.rs
@@ -95,6 +95,8 @@ pub fn container_intermediate_process(
         intermediate_sender
             .close()
             .context("failed to close sender in the intermediate process")?;
+        nix::unistd::setsid()?;
+
         container_init_process(args, main_sender, init_receiver)
     })?;
     // Once we fork the container init process, the job for intermediate process

I am not sure that is the right location, but that works for me. Having it done correctly in --detach is probably much better than what I have done.

[Furisto]

We already call setsid(), but only if we allocate a pseudo terminal. Just looking through the runc code i see that they always seem to call setsid regardless of whether --detach is specified or not. Crun does the same.

[yihuaf]

I will take care of the detached and foreground mode. Although the core of this issue seems to be unrelated to detached anymore lol.