`task_runtime.defaults.docker` must be set, even if all tasks have a defined image

[multimeric]

Here's a reproducible example: https://github.com/multimeric/MiniWdlBug

Basically, if you define task_runtime.defaults but not the docker key, it fails at runtime with ERROR 'docker' :: error: "KeyError", even if all of your tasks have a defined docker image.