ERROR: Can't find Ruby library file or shared library pups

Running ./launcher rebuild mail-receiver today I saw:

Status: Downloaded newer image 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

What does the error message mean, and is it anything to be worried about? Thanks.