A basic image is really easy. It's basically just

Dockerfile

FROM debian  # start with a minimal Linux system. There are probably better options than debian. Some images are made especially for docker (i.e. very minimal and light weight). 
RUN apt install dependencies  # do what ever you need to get your app running. 
RUN echo "options and stuff" >> /etc/a/config/file  # you can also edit system files
COPY . /app  # copy your project into the docker container.
EXPOSE 8080  # doesn't actually do anything, but documents where the app will be listening
CMD server-binary run /app/main.php  # I have actually no idea how php server stuff works

(Docs https://docs.docker.com/reference/dockerfile/)

Then people can run your project with docker.

Edit: checking the readme some small changes would be required. Config.php should read in environment variables and the DB init SQL should be run automatically somehow.

I was looking for something like this a little while ago but couldn't find anything and this looks perfect. My server is mostly docker only so I might try to figure it out.