Consider the use of a Dockerfile linter to enforce best practices

There are various Dockerfile linter tools out there ([hadolint](https://github.com/hadolint/hadolint) comes to mind) that can be used to ensure our Dockerfiles are following best practices. As we continue to evolve our Dockerfiles to take advantage of new features, it's important to ensure we're doing the correct thing which these tools can help with. This is especially important for our sample Dockerfiles that we're offering up to the community as the recommended approach.