In /var/discourse/containers/mail-receiver.yml, what do you have on the base_image: line? It should be:
base_image: discourse/mail-receiver:release
(More info on that here)
In /var/discourse/containers/mail-receiver.yml, what do you have on the base_image: line? It should be:
base_image: discourse/mail-receiver:release
(More info on that here)