Make docker build pipeline always use clean env and actaully work

[bryk]

Previously it didn't always work because my local env was overriding
what was previously baked into image. Now the image bases only on source
code and is brand new each time.

It is also a lot faster when dockerignore excludes more files.

This also moves Dockerfile to build/ directory to avoid confusion from developers about its purpose.

[bryk]

@floreks Can you review?

[codecov-io]

Current coverage is 82.29%

Merging #479 into master will not affect coverage as of 6c42b66

@@            master    #479   diff @@
======================================
  Files           79      79       
  Stmts          644     644       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit            530     530       
  Partial          0       0       
  Missed         114     114       

Review entire Coverage Diff as of 6c42b66

Powered by Codecov. Updated on successful CI builds.

[floreks]

That's really nice.

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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build/Dockerfile, line 23 [r1] (raw file):
Python is not installed anymore

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Comments from the review on Reviewable.io

[bryk]

Merging.

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Review status: 2 of 3 files reviewed at latest revision, 1 unresolved discussion.

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Comments from the review on Reviewable.io