docker-compose.yml tries to mount a volume for the agent without regard to its actual location

[cailyoung]

System info:
macOS 10.14
Docker Desktop

Steps to reproduce:

1. Install buildkite-agent on macOS
2. Click the 'Add to Buildkite' link on this repo
3. Trigger a build

Expected result:
Build succeeds!

Actual result:

$ docker-compose -f docker-compose.yml -p buildkite579c76232f884d959ce27d837d87596d run --name buildkite579c76232f884d959ce27d837d87596d_app_build_3 --rm app /bin/sh -e -c ./scripts/test.sh
--
  | Error response from daemon: Mounts denied:
  | The path /usr/bin/buildkite-agent
  | is not shared from OS X and is not known to Docker.

The macOS install of the agent is not in the folder /usr/bin/, nor does it seem like the mount is actually required for this demo to work; I removed it from the YAML and the pipeline still runs fine.

Would you like a PR with this change? cailyoung@77bafde