Tive o mesmo erro hoje. Nada parou de funcionar da última vez, mas seria bom se alguém pudesse confirmar que está tudo bem.
Stopping old container
+ /usr/bin/docker stop -t 60 mail-receiver
mail-receiver
release: Pulling from discourse/mail-receiver
Digest: sha256:[...]
Status: Image is up to date for discourse/mail-receiver:release
docker.io/discourse/mail-receiver:release
ERROR: Can't find Ruby library file or shared library pups
cd /pups && /pups/bin/pups --stdin