Build debuggable containers

[gsquared94]

Fixes #4370
Add default build args for docker and buildpacks when building for Debug vs non Debug. Also handle skaffold image cache updates.

Redo #4467. No need to extract out ImageOptions since #4598 introduces a Config interface anyways.

User facing changes (remove if N/A)
Dockerfile can expose ARG SKAFFOLD_GO_GCFLAGS and expect Skaffold to supply meaningful values.
Buildpacks auto set GOOGLE_GCFLAGS build options.
Additionally the run mode dev, debug, etc. is exposed as SKAFFOLD_RUN_MODE

[codecov]

Codecov Report

Merging #4606 into master will increase coverage by 0.17%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #4606      +/-   ##
==========================================
+ Coverage   73.20%   73.38%   +0.17%     
==========================================
  Files         339      341       +2     
  Lines       13320    13505     +185     
==========================================
+ Hits         9751     9910     +159     
- Misses       2958     2976      +18     
- Partials      611      619       +8     

Impacted Files	Coverage Δ
pkg/skaffold/build/gcb/docker.go	81.81% <50.00%> (-4.85%)	⬇️
pkg/skaffold/build/local/local.go	63.63% <50.00%> (ø)
pkg/skaffold/docker/image.go	73.36% <50.00%> (-2.64%)	⬇️
pkg/skaffold/docker/build_args.go	70.37% <70.37%> (ø)
pkg/skaffold/build/local/docker.go	78.18% <71.42%> (-2.59%)	⬇️
pkg/skaffold/build/cache/hash.go	73.23% <84.61%> (-2.71%)	⬇️
pkg/skaffold/docker/parse.go	89.94% <84.61%> (-0.43%)	⬇️
pkg/skaffold/build/buildpacks/env.go	89.47% <89.47%> (ø)
pkg/skaffold/build/buildpacks/default_args.go	100.00% <100.00%> (ø)
pkg/skaffold/build/buildpacks/lifecycle.go	81.57% <100.00%> (-1.14%)	⬇️
... and 33 more

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[tejal29]

LGTM!

[tejal29]

Dockerfile can expose ARG SKAFFOLD_GO_GCFLAGS and ARG SKAFFOLD_GO_LDFLAGS and expect Skaffold to supply meaningful values.

Why is this change required? Setting "GO_GC_FLAGS" and "GO_LDFLAGS" in dockerfile is a common practice. Changing this to "SKAFFOLD_GO_GCFLAGS" might break existing user's dockerfile

[gsquared94]

Dockerfile can expose ARG SKAFFOLD_GO_GCFLAGS and ARG SKAFFOLD_GO_LDFLAGS and expect Skaffold to supply meaningful values.

Why is this change required? Setting "GO_GC_FLAGS" and "GO_LDFLAGS" in dockerfile is a common practice. Changing this to "SKAFFOLD_GO_GCFLAGS" might break existing user's dockerfile

I'm not very opinionated about this. @briandealwis had suggested prefixing SKAFFOLD_, I think as a way to distinguish that it's an environment variable that we pass in explicitly. The user may set regular GO_LDFLAGS and GO_GCFLAGS in his Dockerfile to any arbitrary value and ignore skaffold supplied values entirely. I'm not sure why you think that it would break them though.

[gsquared94]

@briandealwis @tejal29 @nkubala PTAL.

[briandealwis]

One question and a few optional changes