Simplify fake docker api

[dgageot]

No description provided.

[codecov-io]

Codecov Report

Merging #1424 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
- Coverage   44.77%   44.76%   -0.02%     
==========================================
  Files         111      111              
  Lines        4554     4553       -1     
==========================================
- Hits         2039     2038       -1     
  Misses       2310     2310              
  Partials      205      205

Impacted Files	Coverage Δ
pkg/skaffold/docker/image.go	59.18% <100%> (ø)	⬆️
pkg/skaffold/build/local/local.go	54.34% <50%> (-0.98%)	⬇️

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