Build out unshare Executor and Instance API

To allow us to run lintian, piuparts and blhc under unshare, we need to implement the rest of the Executor and Instance API. (#219 (closed)) Currently most of the low-level functions are NotImplemented: https://salsa.debian.org/freexian-team/debusine/-/blob/devel/debusine/tasks/executors/unshare.py

How we envision unshare instances to work:

• An unshare instance is an extracted system-tarball in a temporary directory.
• No init system is run in the instance.

Requirements:

• Creating the instance should extract the tarball into a temporary directory.
• Deleting the instance (including GC) clean up the temporary directory. The directory can be deleted from within unshare, the way autopkgtest-virt-unshare does.
• Starting the instance is a noop.
• Stopping the instance should be a noop.
• File push should install a file with appropriate permissions in the container. Maybe using the ownership of the parent directory? This may require passing the file to a cat inside the unshare.
• File pull should copy a file out of the container.
• Run should execute the provided command as an argument to unshare. autopkgtest-virt-unshare uses --map-auto, --map-root-user, --user, --mount. I guess we should do the same.
  • If possible: Use --kill-child and --pid to clean up processes at the end of the run.