Hi! I wanted to share that I have release a 0.1 version of a new project called Dockerfile+ (or Dockerfile-plus). This is a custom buildkit frontend which will add interesting new syntax for Dockerfiles. This first release just adds a single new command: INCLUDE+. This command will import the contents of another file into your Dockerfile.
If you have a buildkit-enabled Docker daemon (18.09+) you can use this new frontend by adding this line as the first line of your Dockerfile:
# syntax = edrevo/dockerfile-plus
More info at https://github.com/edrevo/dockerfile-plus