As outlined in this GitHub issue, MongoDB 3.0.6 Docker container contains some changes that might affect the container’s functionality on CircleCI.
For now the workaround we suggest is to pin the MongoDB container version to 3.0.5:
docker run --name db -d mongo:3.0.5 mongod
UPDATE: The issue has been fixed on the container side, everything should be working again without the need to pin the container version.