When puma was introduced in #8392 the default listen configuration was
set to only localhost, which makes sense for most development setups,
but when run within docker, it needs to listen on all IPs so the port
can be forwarded to be accessable outside of docker.
Because the new default makes sense without docker, I overwrite the
option with a environment variable only in the docker-setup. This also
ensures that it always contains the right value needed for the
docker-setup to work, no matter what was configured outside of docker.
