This can be done manually as explained here. But I wanted to do this on one fell swoop, so this command worked:

echo "lxc.mount.entry = /src/path/i/wanted/to/share  /var/lib/lxc/container-name/rootfs/mnt none bind 0 0" | sudo tee -a /var/lib/lxc/container-name/config

If done frequently, a function may be useful. I’m too lazy to write that now but I’ll add it later.