Add fixes to Dockerfile

[amrit110]

This PR adds a couple of fixes to Dockerfile.

1. Fix typo in url to git repo in Dockerfile, while cloning
2. Fix pip install command to use requirements file in top level dir (requirements/build.txt does not exist)

Fixes #595

[CLAassistant]

All committers have signed the CLA.

[Junjun2016]

Hi @amrit110
Thanks for your kind PR.

[codecov]

Codecov Report

Merging #607 (6c4e990) into master (8f8abe3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #607   +/-   ##
=======================================
  Coverage   85.45%   85.45%           
=======================================
  Files         101      101           
  Lines        5220     5220           
  Branches      840      840           
=======================================
  Hits         4461     4461           
  Misses        586      586           
  Partials      173      173           

Flag	Coverage Δ
unittests	85.45% <ø> (ø)

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