Better Dockerfiles

[gchqdev227]

This PR updates the recommended (and generated) Dockerfiles to be more inline with current best practices. The previous versions were in no way insecure, but it makes sense to make use of Docker secrets.

Summary of changes

• Dockerfiles are now generated with build secrets instead of multi-stage builds
• Added new commands to the dockerfile CLI command
• Better documentation on how the Dockerfiles are made up
• Dockerfiles are generated from a number of Instruction classes in order to dynamically build them with less code

Most of the added lines correspond to the new test cases, where each one tests against a hardcoded Dockerfile.

I recommend we do not squash these commits.