fix: make useBuildkit field nullable across all config versions

[gsquared94]

fix: setting useBuildkit: false actually turns off buildkit for docker builder

Related: #4936

Description

Currently setting useBuildkit: false in the skaffold.yaml config doesn't actually disable buildkit. This PR fixes that by setting DOCKER_BUILDKIT =0 env variable when useBuildkit: false is set in the config.

In pkg/skaffold/build/docker/docker.go :

-	if b.useBuildKit {
-		cmd.Env = append(cmd.Env, "DOCKER_BUILDKIT=1")
+	if b.useBuildKit != nil {
+		if *b.useBuildKit {
+			cmd.Env = append(cmd.Env, "DOCKER_BUILDKIT=1")
+		} else {
+			cmd.Env = append(cmd.Env, "DOCKER_BUILDKIT=0")
+		}
	}

This change in itself would however break backwards compatibility since for previous versions of the config even if useBuildkit property is not set, it'd default to false and now start disabling buildkit which is not how it worked previously (so that bug essentially became a feature, that if useBuildkit is unspecified it should use the Docker default).

So we additionally make the config change to all config versions, so that a missing useBuildkit field can be identified separately from an explicitly set useBuildkit: false

User facing changes (remove if N/A)
There are no user facing changes other than if users were setting useBuildkit: false in the config, skaffold will now actually disable buildkit.

Follow-up Work (remove if N/A)

Fix #4936 after this is merged.

Changes discussed with @briandealwis

The make checks will be failing since we generally prohibit making changes to already released configs. But this PR only changes a single field from bool to *bool.

[codecov]

Codecov Report

Merging #6612 (751617e) into main (290280e) will decrease coverage by 0.49%.
The diff coverage is 87.30%.

@@            Coverage Diff             @@
##             main    #6612      +/-   ##
==========================================
- Coverage   70.48%   69.99%   -0.50%     
==========================================
  Files         515      520       +5     
  Lines       23150    23603     +453     
==========================================
+ Hits        16317    16520     +203     
- Misses       5776     6001     +225     
- Partials     1057     1082      +25     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/flags.go	89.00% <ø> (-1.82%)	⬇️
cmd/skaffold/skaffold.go	0.00% <ø> (ø)
pkg/skaffold/build/buildpacks/logger.go	0.00% <ø> (ø)
pkg/skaffold/config/options.go	100.00% <ø> (ø)
pkg/skaffold/debug/apply_transforms.go	22.64% <0.00%> (-0.44%)	⬇️
pkg/skaffold/debug/transform.go	95.91% <ø> (ø)
pkg/skaffold/docker/logger/log.go	22.64% <0.00%> (-0.44%)	⬇️
pkg/skaffold/kubectl/exec.go	100.00% <ø> (ø)
pkg/skaffold/schema/latest/v1/config.go	62.16% <ø> (ø)
pkg/skaffold/schema/latest/v2/config.go	5.40% <ø> (ø)
... and 86 more

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[briandealwis]

Worth a comment here to say that we ignore the useCLI setting if buildkit is requested as we currently use the Docker CLI to use buildkit. We could consider a different approach in the future.

[briandealwis]

@nkubala fyi: this PR changes previously-published config schemas to turn useBuildkit from a boolean into an optional field.