Volumes: Add a short-form specifier

[JayH5]

I'm not entirely certain about this design, but I think it definitely reduces boilerplate code when defining volumes to mount in a lot of cases.

The reason I'm not certain about this design is that it's one I've half made-up. It sits somewhere between docker-compose's short and long form. In docker-compose the long-form is kind of similar to the interface provided by the Docker Python SDK. The short-form that I'm inventing here is closer to the docker-compose short-form, but still shares some properties with the long-form.

In docker-compose the short-form is like:

volumes:
  - my_volume:/mnt:rw

whereas here we keep things as a dict so that the volumes themselves don't have to be specified as strings, just the mountpoint:

{ my_volume: '/mnt:rw' }

Hope this makes sense...

[codecov[bot]]

Codecov Report

Merging #23 into develop will increase coverage by <.01%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #23      +/-   ##
===========================================
+ Coverage     99.5%   99.51%   +<.01%     
===========================================
  Files           20       20              
  Lines         1806     1837      +31     
  Branches       130      131       +1     
===========================================
+ Hits          1797     1828      +31     
  Misses           7        7              
  Partials         2        2

Impacted Files	Coverage Δ
seaworthy/dockerhelper.py	100% <100%> (ø)	:arrow_up:
seaworthy/tests-core/test_dockerhelper.py	100% <100%> (ø)	:arrow_up:

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