# No auth
docker run -e DISPLAY=host.docker.internal:0 xeyes
# For auth...
# Get cookie
xauth list
$HOSTNAME:0 MIT-MAGIC-COOKIE-1 $COOKIE
...
# Add entry in container (somehow; could mount or run this command while inside)
xauth add host.docker.internal:0 MIT-MAGIC-COOKIE-1 $COOKIE
NO way to use graphical Steam installation's authenticated session with steamcmd: must prompt for re-login.