Help running Dredd with a Rails backend

We’ve got an application running Rails 5 beta 3 that has a Dredd test suite. When we run a Circle CI build, Dredd fails with this error:

info: Configuration dredd.yml found, ignoring other arguments.
info: Starting server with command: RAILS_ENV=test bin/rails server -p 3009 -b
info: Waiting 3 seconds for server command to start...
error: Server command failed, exiting Dredd...
info: Sending SIGTERM to the backend server
info: Backend server exited

dredd returned exit code 2

This same command dredd, when run locally, fires up the Rails server and runs the Dredd suite successfully.

I tried restarting the Circle CI build with SSH access and was able to confirm that the command to start the server (RAILS_ENV=test bin/rails server -p 3009 -b works as expected.

Has anyone encountered anything like this? Thanks in advance for any advice anybody might have.