Share the Go build cache when building in Dockerfiles (#1853)

On my machine this makes image rebuilds go from ~5m to 1.5s.

This also required setting DOCKER_BUILDKIT=1 in integration test image
builds.

I also took the opportunity to bump the Go version used in those tests
to Go 1.17 to match the image build processes, and tidied up the
workflow files a bit too (renaming for consistency, typos, whitespace,
etc.)

scripts/integration-test.sh

@@ -20,7 +20,6 @@ IMAGE_REPO="${IMAGE_REPO:-gcr.io/kaniko-test}"
 
 docker version
 
-# Sets up a kokoro (Google internal integration testing tool) environment
 echo "Running integration tests..."
 make out/executor
 make out/warmer