Add dockerignore

[pjonsson]

This avoids bringing in various
local files into the docker build
context, so the docker cache will
be tainted less often, improving
the performance of image rebuilds.

If the docker cache gets in the way,
one can still use docker build --no-cache.

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📚 Documentation preview 📚: https://datacube-explorer--597.org.readthedocs.build/en/597/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.32%. Comparing base (aa0d9f3) to head (ddb3345).
Report is 118 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #597      +/-   ##
===========================================
+ Coverage    86.29%   86.32%   +0.03%     
===========================================
  Files           26       26              
  Lines         3451     3466      +15     
===========================================
+ Hits          2978     2992      +14     
- Misses         473      474       +1     

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[omad]

Thanks, wise.