Case
We want to copy only a specific directory of the host into the container.
Solution
If you want to copy only a specific directory instead of mounting it to the container, use the following command.
tar cf - /path/on/host | lxc exec container -- tar xvf - -C /path/on/target
It can be omitted if it is okay to expand to the current directory in the container with the same directory name as the source -C
tar cf - /path/on/host | lxc exec container -- tar xvf -