Adding a first version of dockerized development environment.

[DainDwarf]

Closes #139

This adds a development docker environment that should let you get to it immediately.

Successful PR Checklist:

• Tests
• Documentation (optionally: run spell checking)
• Had a good time contributing? (if not, feel free to give some feedback)

[DainDwarf]

Still TODO:

• Find a way to play migrations if needed, before starting the worker
• Add documentation!

[codecov-io]

Codecov Report

Merging #144 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #144   +/-   ##
=====================================
  Coverage     100%   100%           
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  Files          21     21           
  Lines         835    835           
  Branches       85     85           
=====================================
  Hits          835    835

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[k4nar]

👍

[elemoine]

Ok, testing now…

[elemoine]

This is now ready for review again. As discussed with @DainDwarf and @ewjoachim I created in the CONTRIBUTING guide a section dedicated to working with Docker, the goal being to avoid clobbering the CONTRIBUTING guide with Docker-related stuff that developers not using Docker don't care about.