Some manual setup still required.
Add packages with:
docker exec --tty -u build ulrepo-stuff_aur-builder_1 aur sync -d ulrepo --root /var/cache/pacman/ulrepo -ncT --noview <package-name(s)>
Remove packages with:
docker exec --tty -u build ulrepo-stuff_aur-builder_1 repo-remove /var/cache/pacman/ulrepo/ulrepo.db.tar <package-name(s)>
The host must run kernel version 3.15 or higher, see: https://bbs.archlinux.org/viewtopic.php?id=232682