Fixes https://groups.google.com/d/topic/nomad-tool/B3Uo6Kns2BI/discussion
The test used old local copies of Docker structs and appeared to be testing an outdated approach to task config decoding. Updated to use real Docker structs so we can do end-to-end unit testing of real Docker task configs.
No need to pass in the entire eval context
I wanted a different color bikeshed, so I get to paint it
