Docker image for even easier local deployment

[ihyoudou]

I thought it would be cool to add a very quick and simple method to deploy, so anyone could have their own, local version of frogfind running on their computer or even SBC like Raspberry Pi.

The PR also include Github Action workflow to automatically build multiarch Docker image and push it to Github Registry.

I know that this project is very simple to install as-is, as all you need to have is a working web server with composer, but I think it still might be useful for someone.

[popey]

Thanks for providing this additional option @ihyoudou !

I built and tested the docker image on my server, and it's running well. I'll leave it up for a while in case anyone else wants to poke at it, to make sure it behaves as expected.

I didn't test the actions, but it all looks sensible!