Get digest on push and imageID on build

[dgageot]

This code tries to retrieve the digest/imageID immediately after the push/build.

Will enable a few simplifications in builders' code

[codecov-io]

Codecov Report

Merging #1428 into master will decrease coverage by 0.12%.
The diff coverage is 51.06%.

@@            Coverage Diff             @@
##           master    #1428      +/-   ##
==========================================
- Coverage   44.78%   44.66%   -0.13%     
==========================================
  Files         111      111              
  Lines        4551     4579      +28     
==========================================
+ Hits         2038     2045       +7     
- Misses       2308     2326      +18     
- Partials      205      208       +3

Impacted Files	Coverage Δ
pkg/skaffold/build/local/local.go	54.34% <0%> (ø)	⬆️
pkg/skaffold/build/local/jib_maven.go	38.59% <0%> (ø)	⬆️
pkg/skaffold/build/local/bazel.go	37.5% <0%> (ø)	⬆️
pkg/skaffold/build/local/jib_gradle.go	21.87% <0%> (ø)	⬆️
pkg/skaffold/build/local/docker.go	17.64% <50%> (-3.41%)	⬇️
pkg/skaffold/docker/image.go	56.25% <57.5%> (-2.94%)	⬇️
pkg/skaffold/docker/auth.go	38.09% <0%> (-14.29%)	⬇️

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