I did a git clone and have been following the instructions and it seems I am failing to build.
Instructions:
Instructions

I ran:
./steam-runtime/setup_docker.sh --beta amd64 --extra-bootstrap=./proton/steamrt-bootstrap.sh steam-proton-dev

and at the end I got:

Step 8/20 : RUN 	set -xe; 	if [ -n "${proxy}" ]; then 		echo "configure proxy"; 		echo "export http_proxy=${proxy}" >> /etc/bash.bashrc; 		echo "export https_proxy=${proxy}" >> /etc/bash.bashrc; 		echo "export ftp_proxy=${proxy}" >> /etc/bash.bashrc; 		echo "Acquire::http::proxy \"${proxy}\";" > /etc/apt/apt.conf.d/01proxy; 		echo "Acquire::https::proxy \"${proxy}\";" >> /etc/apt/apt.conf.d/01proxy; 	fi; 	if [ -n "${no_proxy}" ]; then 		echo "export no_proxy=${no_proxy}" >> /etc/bash.bashrc; 		oldIFS="$IFS"; 		IFS=","; 		for nope in ${no_proxy}; do 			case "$nope" in 				(.*) 					;; 				(*) 					echo "Acquire::http::proxy::${nope} \"DIRECT\";" >> /etc/apt/apt.conf.d/01proxy; 		echo "Acquire::https::proxy::${nope} \"DIRECT\";" >> /etc/apt/apt.conf.d/01proxy; 					;; 			esac; 		done; 		IFS="$oldIFS"; 	fi
 ---> Running in e2cfab95578d
OCI runtime create failed: this version of runc doesn't work on cgroups v2: unknown

real	0m28.535s
user	0m0.645s
sys	0m0.481s
Script done, file is /tmp/steam-runtime-setup-chroot-Vm8.log
[steam-runtime-setup-chroot-Vm8.log](https://github.com/ValveSoftware/Proton/files/4536111/steam-runtime-setup-chroot-Vm8.log)

Full log attached
steam-runtime-setup-chroot-Vm8.log