I pushed this patch that allows you to set global docker network options. This patch is available in the latest version of the runner image. You can set global network options as follows:
DRONE_RUNNER_NETWORK_OPTS=com.docker.network.drive.mtu:1442
This patch should work based on the docker documentation, but I do not have an environment that I can use to test this. If you experience any issues we would kindly ask that you send a pull request that improves this code.