Add highlighting for Dockerfile

[candrews]

Is your feature request related to a problem? Please describe

It would be great to support Dockerfile syntax highlighting.

Describe the solution you'd like

[GitHub supports Dockerfile syntax highlighting](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks] so it's a great way to see what I think this Wordpress plugin should also do:

# syntax=docker/dockerfile:1

FROM ubuntu:22.04
COPY . /app
RUN make /app
CMD python /app/app.py

Describe alternatives you've considered

Additional context

Add any other context or screenshots about the feature request here.