[How-To] Wait for LUKS unlock before start Docker containers

Problem

I have my hard drives encrypted with LUKS. Passphrase to unlock drives can be only typed in in the OMV WebUI. I don't want to use auto unlock. Encrypted hard drives have directories that are in use by some of mine Docker containers (nextcloud, transmission). I don't want let those containers to start before the unlock. It will fail, produce some garbage on the filesystem and then I will have manually restart those containers.

Fix

We need script that will wait for the unlock and only then start Docker service

Script

You can copy the code from my GitHub (wait-for-unlock.sh) and put it to /usr/local/bin/ or just download it directly from your device:

sudo wget -P /usr/local/bin/ https://raw.githubusercontent.com/Virusmater/OdroidXU4-Cloudshell2-OMV/master/usr/local/bin/wait-for-unlock.sh
sudo chmod a+x /usr/local/bin/wait-for-unlock.sh

Docker Service

Make an override of an existing configuration

sudo systemctl edit docker

and add the script execution before the start

[Service]
ExecStartPre=/usr/local/bin/wait-for-unlock.sh

that will create file /etc/systemd/system/docker.service.d/override.conf with that configuration. You can also check it on GitHub (override.conf)

Final

Script wait-for-unlock.sh is quite dummy and checks only for "/dev/mapper/sda-crypt" device. Any other ideas and PRs are welome!

From now on Docker service will wait for the unlock before starting any containers.

Just find out that it will hang upgrade of the package docker-ce because it waits for the service to start. So before the upgrade you should unlock your LUKS volume.