Local builders return a digest

[dgageot]

Each builder is in charge of returning the digest of what it built.
digest is still digest|imageID though.

Signed-off-by: David Gageot david@gageot.net

[balopat]

LGTM

[codecov-io]

Codecov Report

Merging #1374 into master will increase coverage by 0.03%.
The diff coverage is 21.42%.

@@            Coverage Diff             @@
##           master    #1374      +/-   ##
==========================================
+ Coverage   44.88%   44.91%   +0.03%     
==========================================
  Files         114      114              
  Lines        4919     4909      -10     
==========================================
- Hits         2208     2205       -3     
+ Misses       2485     2481       -4     
+ Partials      226      223       -3

Impacted Files	Coverage Δ
pkg/skaffold/build/local/jib_maven.go	38.59% <0%> (ø)	⬆️
pkg/skaffold/build/local/jib_gradle.go	21.87% <0%> (ø)	⬆️
pkg/skaffold/build/local/docker.go	21.05% <100%> (ø)	⬆️
pkg/skaffold/build/local/local.go	55.31% <100%> (+3.53%)	⬆️
pkg/skaffold/build/local/bazel.go	37.5% <25%> (+0.76%)	⬆️

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[dgageot]

@balopat Thank you, I put a wip label because I need to triple check if the Bazel builder still works