Document that running user needs to be in docker group

[david4096]

Add a note to the documentation that the user running workflow-service needs to be in the docker group in order to run tests.

[DailyDreaming]

This should be addressed in the code. If you try to add, say, your current user name to the docker group, but you are running on aws, it won't let you because that user doesn't exist on that machine.

[DailyDreaming]

It works on localhost:8080 just fine because they see the same users and groups, but that is not the case if the wes-server is hosted remotely.