Add highlighting for Dockerfile

### 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:
```dockerfile
# 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.