fixes skaffold version in the released docker image

[balopat]

proof by manual testing, that I also added to the DEVELOPMENT.md

$ docker run gcr.io/<myproject>/skaffold:testrelease_v2 skaffold version
testrelease_v2

fixes #906.

[codecov-io]

Codecov Report

Merging #933 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #933   +/-   ##
=======================================
  Coverage   43.58%   43.58%           
=======================================
  Files          62       62           
  Lines        2611     2611           
=======================================
  Hits         1138     1138           
  Misses       1354     1354           
  Partials      119      119

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