Default pipeline type

[bradrydzewski]

Here is a brief explanation of purpose of the type field:

The Pipeline object defines a Continuous Integration and Continuous Delivery pipeline. The type attribute defines the runner that should be used when executing the pipeline. Drone offers support for a variety of runners.

https://docs.drone.io/pipeline/configuration/#pipelines

And here is a brief explanation of why it is not technically a required field, but should be treated as such when creating new pipelines:

@bradrydzewski

The type attribute should be considered a required field. The reason it defaults to docker is because we needed to support legacy yaml files that were created before Drone had the concept of multiple runner types. However, you should not rely on this legacy behavior when creating new pipelines since, at some point, we will remove this legacy default and will fail a pipeline if the type attribute is missing.