proposed PR: automate dockerbuild and push to dockerhub

Hi,

When I find a project like this i tend to fork, make my own multi-architecture containers using github workflows and push to dockerhub as that then becomes easy for me to use down the line.

I am in middle of doing this, i saw you like discussing things before a PR is raised (and that i need to raise it against the development branch).

1) when I get this working would you like me to submit the PR? (it will include doc changes and file changes to docker-compose example, the docker file and new workflow files, alternately I can keep it mine, but may not keep it that upto date.)
2) if so it would require you to create your own secrets in the master project for your docker repo
3) wow this a large image - are all those reqs really required - esp the dev version of python etc - i find i can reduce image size by switching over to run time version only.
4) what does the gateway env var do (i am building the container before i have even run the app, lol, so maybe that will become clear to me, so apologies if a dumb question.

(nice project, looking forward to converting 4 devices i have)

Scyto