docs: Developing the docker environment, fixes #81

[coveralls]

Pull Request Test Coverage Report for Build 874251949

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 42 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 70.178%

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Files with Coverage Reduction	New Missed Lines	%
standards_lab/api/views.py	42	51.7%
<!--	Total:	42		-->

Totals
Change from base Build 856824500:	0.1%
Covered Lines:	433
Relevant Lines:	617

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💛 - Coveralls

[odscjames]

Tho could be a bit more explicit about some other things:

If change python, don't have to reubilt, just down & up

[odscjames]

how to run tests in docker local env

Tho this is in another docs file, so maybe just a link

[odscjames]

Also would be good: If you want to temporarily put print/logging calls in your Python code and see them appear in the logs that scroll on your screen, how do you do that?

A: import warnings, warnings.warn

[rhiaro]

Updated based on feedback, thanks @odscjames @Bjwebb @michaelwood

[odscjames]

Just realised one thing we missed - we need to link to this new file in the index. I've just added a commit to do that.