If you already have a GNU/Linux system, lxc would be the easiest to setup.

Install required packages

Install lxc using this command.

sudo apt-get install lxc

If you have Ubuntu 14.04/trusty then install lxc from backports

sudo apt-get -t trusty-backports install lxc

Now install some necessary packages for networking support for the container.

sudo apt-get install -qy libvirt-clients libvirt-daemon-system iptables ebtables dnsmasq-base # if libvirt-client is not available, try libvirt-bin

Networking setup

Check status of libvirt daemon (service)

systemctl status libvirtd 

and start if not running

systemctl start libvirtd 

Now start the networking service using

sudo virsh net-start default
sudo virsh net-autostart default

Check LXC#Network_setup_in_buster for buster specific changes.

Create sid container

Now create the container named debian-sid

sudo lxc-create -n debian-sid -t download -- --dist debian --release sid

You might have to use lxc-attach instead of lxc-console to connect to the container. After attaching yourself you could set your root password using passwd.

Using the created container

Before connecting to the container, you might want to start the container

sudo lxc-start -n debian-sid

To connect to the container

sudo lxc-attach -n debian-sid

This is assuming that you have named your container 'debian-sid' as per the previous instructions.

Create normal user

Create a new user with adduser and switch to that user with su - <username>. See https://www.digitalocean.com/community/tutorials/how-to-add-and-delete-users-on-debian-8. Easiest way to create a user with permission to run sudo would be:

useradd -m -g sudo <username>

It is worth noting that you need to install sudo as it does not come default. Also the -m tag creates a home folder for the user, this is not trivial and can be skipped.

Note: You can install sbuild inside the lxc container for clean builds.