refactor: avoid twice parse Dockerfile

[orisano]

avoid twice parse Dockerfile

[tejal29]

@orisano Can you please explain how this is avoiding parsing dockerfile twice?
Also please add some tests to make sure dockefile is parsed just once.

[antechrestos]

@orisano @tejal29 I can contribute if needed. I understand what @orisano did

In the beginning of DoBuild, we have

stages, err := dockerfile.Stages(opts)

then couple of lines later

crossStageDependencies, err := CalculateDependencies(opts)

then at the beginning of CalculateDependencies: stages, err := dockerfile.Stages(opts)

Hence, making CalculateDependencies private and passing stages as parameters is a good option.

I can contribute if needed and find a way to enforce the face that the file gets loaded only once.

[antechrestos]

@orisano This pull request made the job