Fix dockerfile resolution

[briandealwis]

Fixes: #4243

Description

pkg/skaffold/docker.NormalizeDockerfilePath attempted to handle the case where a user specified their dockerfile including the context directory:

- image: my/image
   context: project
   docker:
     dockerfile: "project/Dockerfile"

The code uses strings.HasPrefix to check if the dockerfile path included the context directory:

skaffold/pkg/skaffold/docker/dependencies.go

Lines 33 to 42 in 5444e54

	func NormalizeDockerfilePath(context, dockerfile string) (string, error) {
	if filepath.IsAbs(dockerfile) {
	return dockerfile, nil
	}
	if !strings.HasPrefix(dockerfile, context) {
	dockerfile = filepath.Join(context, dockerfile)
	}
	return filepath.Abs(dockerfile)
	}

This breaks on @ sujit-kamireddy's example in #4243 where the context directory is a prefix of Dockerfile:

- image: my/image
   context: Dock

(Although this situation may seem far-fetched, a context with Docker is not so unexpected.)

This code should first check if the context+dockerfilePath exists and only then use just the dockerfilePath alone.

User facing changes (remove if N/A)

There should be no changes.

[codecov]

Codecov Report

Merging #4260 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
pkg/skaffold/docker/dependencies.go	75.34% <100.00%> (+2.73%)	⬆️
pkg/skaffold/kubernetes/watcher.go	87.50% <0.00%> (-12.50%)	⬇️
...affold/kubernetes/portforward/kubectl_forwarder.go	63.41% <0.00%> (-4.88%)	⬇️
...ffold/kubernetes/portforward/resource_forwarder.go	84.31% <0.00%> (-3.45%)	⬇️
...g/skaffold/kubernetes/portforward/pod_forwarder.go	82.75% <0.00%> (-3.18%)	⬇️
pkg/skaffold/server/server.go	53.65% <0.00%> (ø)
pkg/skaffold/deploy/helm.go	80.27% <0.00%> (+0.02%)	⬆️
...affold/kubernetes/portforward/forwarder_manager.go	30.00% <0.00%> (+1.42%)	⬆️
pkg/skaffold/kubernetes/log.go	36.84% <0.00%> (+2.69%)	⬆️
...g/skaffold/kubernetes/portforward/entry_manager.go	96.49% <0.00%> (+2.87%)	⬆️
... and 2 more