ci(deploy): add digest to docker image to ensure uniqueness

[ewanharris]

The dokku docs state we can use the digest of the image if tags are reused (search for Triggering a build with the same arguments multiple times will result in Dokku exiting 0 early as there will be no changes detected)

This is output from the docker build action in the outputs and as the deploy_docker_image is just passed to git:from_image in the docker image used by the action I'm fairly sure this should be plumbed together ok.

[cb1kenobi]

Let's give it a go