[build] Use --short in GITSHA calculation

[wesleyk]

git rev-parse HEAD won't return an 8 character SHA by default, which we want for our docker tags

Thanks for contributing to the M3DB Operator! We'd love to accept your contribution, but we require that most issues
outside of trivial changes such as typo corrections have an issue associated with the pull request. So please open an
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[codecov]

Codecov Report

Merging #288 (97f73ef) into master (d9a13c6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #288   +/-   ##
=======================================
  Coverage   76.03%   76.03%           
=======================================
  Files          32       32           
  Lines        2541     2541           
=======================================
  Hits         1932     1932           
  Misses        430      430           
  Partials      179      179           

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