First, you need to make sure the UID of the root user in the container (if you're using root) maps to the UID of the owner of the shared directory:

johannes@host: ls -ln <source_dir>
drwsrwsrwt 7 1000 1000  4096 Oct  9 11:33 .

In this case, it's 1000

Then, configure the container to use that same UID for the root user

johannes@host: sudo lxc config set <container> raw.idmap "both 1000 0"

You'll need to restart the container for these changes to take effect

johannes@host: sudo lxc restart <container>

Then, add the folder as a device to the container

johannes@host: sudo lxc config device add dev <container> disk source=/<source_dir> path=/shared/<some_folder_name>

Double check that the permissions are set correctly in the mounted directory

root@dev: ls -la /shared
total 8
drwxr-xr-x 1 root root   54 Oct  9 15:31 .
drwxr-xr-x 1 root root  256 Oct  9 15:30 ..
drwsrwsrwt 9 root root 4096 Oct  9 15:36 pass-import-1pux

That looks good! You're now able to read/write that folder from inside the container.