Build refuses to run since adding docker into circle.yml file

My build used to work, but since I’ve attempted to add docker into it I now get the message…

Not run
Avoid queues by adding containers, skipping redundant builds (through project settings or configuring your circle.yml)

Sorry! Your build didn’t run because it looks like you haven’t selected an OS X plan yet. If you have any questions, please free to contact us in support.

I only have one project building on CircleCI, so I’m still within the free tier.

Does the use of Docker not come within the free tier?