Improve caching

[priyawadhwa]

This PR:

• makes sure caching can still work without a local daemon (check remote registry if the image exists even without local Docker)
• makes sure caching works with kaniko & docker in GCB (tag images with imageName:cacheHash as well in the cloudbuild.yaml and args to kaniko)

TODO:

• make sure caching works with Jib in GCB / Jib locally

Continues work on #1740

[codecov-io]

Codecov Report

Merging #1755 into master will decrease coverage by 0.05%.
The diff coverage is 48.64%.

@@            Coverage Diff            @@
##           master   #1755      +/-   ##
=========================================
- Coverage   47.05%     47%   -0.06%     
=========================================
  Files         130     130              
  Lines        6241    6270      +29     
=========================================
+ Hits         2937    2947      +10     
- Misses       3003    3021      +18     
- Partials      301     302       +1

Impacted Files	Coverage Δ
pkg/skaffold/build/kaniko/run.go	0% <0%> (ø)	⬆️
pkg/skaffold/plugin/environments/gcb/docker.go	100% <100%> (ø)	⬆️
pkg/skaffold/plugin/environments/gcb/desc.go	55.1% <41.66%> (-1.42%)	⬇️
pkg/skaffold/build/cache.go	55.98% <47.05%> (+0.42%)	⬆️
pkg/skaffold/plugin/environments/gcb/types.go	0% <0%> (ø)	⬆️

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