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.