Pass buildArgs to Kaniko

[JoelPM]

If buildArgs are present in skaffold.yaml for the Docker configuration pass them through to kaniko. Addresses #821

This is likely not idiomatic golang, happy to update based on feedback.

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[JoelPM]

CLA is signed.

[googlebot]

CLAs look good, thanks!

[r2d4]

you can use append(args, fmt.Sprintf("--build-arg=%s=%s", k, *v)

[r2d4]

https://tour.golang.org/moretypes/15

[JoelPM]

Updated based on feedback. That's definitely cleaner.

[JoelPM]

@r2d4 - Made the changes you suggested, any other changes you'd like to see?

[r2d4]

LGTM Thanks!!

[dgageot]

LGTM with a nit

[dgageot]

Can be replaced with if len(artifact.DockerArtifact.BuildArgs) == 0

[JoelPM]

Good point @dgageot, nit addressed.