If you mean an external directory within immich, then answering this is out of scope of the Armbian project. We provide a fairly easy way to install the software, but everything beyond is userspace.
If you mean sharing a directory from the host into the docker container, then here you can check how armbian-config installed immich:
https://github.com/armbian/configng/blob/37f091998d74b059c4302a91ca1f2c06a908297f/tools/modules/software/module_immich.sh#L16