Do not leak information from the host system

[giordano]

Sandbox.jl currently leaks some information from the host system. You can use the basic example in the README to start a shell:

using Sandbox

config = SandboxConfig(
    Dict("/" => Sandbox.alpine_rootfs());
    stdin=Base.stdin,
    stdout=Base.stdout,
    stderr=Base.stderr,
)
with_executor() do exe
    run(exe, config, `/bin/sh`)
end

and here run the commands uname -n and hostname: they'll show the hostname of the host system. Also, the command env will show some environment variables, not many, which include SUDO_COMMAND, SUDO_USER (in case you're using a privileged runner) and SHELL from the host system.

It'd be nice to not leak this information from the outside world inside the sandbox. For example, Elliot suggested we can optionally create a new UTS namespace to set the hostname: https://man7.org/linux/man-pages/man7/uts_namespaces.7.html, https://man7.org/linux/man-pages/man2/gethostname.2.html