Ran the tests in src/github.com/drone/drone/remote/github with my webhook payload. The Github webhook parsing worked.
Found the error template return fmt.Errorf("Cannot unmarshal '%v' to type %T into a string value", name, name)
in src/github.com/drone/drone/vendor/github.com/docker/libcompose
I fear the error is coming form the layer where Drone tries to create Docker volumes/networks etc.
I presume this is for Drone 0.8 because Drone 1.0 does not use libcompose? I would recommend upgrading to 1.0. If you can reproduce this problem in 1.0 let me know and I can investigate further.