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
Arch/Manjaro users see https://wiki.archlinux.org/index.php/Linux_Containers
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
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.
lxc-attach would not setup the tty session for other users and ask pass, what that means is, no sudo, if you switch using su - <username> (see next step). Use lxc-console if you want to use sudo.
See http://blog.scottlowe.org/2013/11/25/a-brief-introduction-to-linux-containers-with-lxc/ for more info on using lxc.
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.