add -insecure flag and relevant tests

This first iteration provides a `-insecure` flag for the `docker run` command.
A container running with this flag:
- has access to all the devices, instead of being restricted to a small, sane set of devices;
- retains all the capabilities of the host.
It means that this container will be able to `mknod`, `mount`, etc.

Later, if it's deemed necessary, we can add an `INSECURE` directive to the builder, to mark the fact that a specific container requires extended privileges; but for now, I think that this is a very helpful starting point!