Refactor utility `MkBuildContext` to the more generic `archive.Generate`

This facilitates refactoring commands.go.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4102 from shykes/separate-mkbuildclient

Refactor utility `MkBuildContext` to the more generic `archive.Generate`