Add docs for custom docker image

[alexander-schranz]

I'm missing something like this in the docs.

[dunglas]

Can't we merge this with the existing docs about Docker in a "Docker" section?

[alexander-schranz]

@dunglas Yeah I was not sure to mix them up. I think the currenlty docker.md is focused for Contributers or Maintaincers of FrankenPHP not about the one using it.

I even think worker.md linked as The worker mode is misleading and should be called Getting Started instead or Using FrankenPHP and then maybe add it there?

Also I personally more a fan to have one bigger doc file instead of splitting the docs into multiple files make searching for specific things easier and think why also Symfony merged a lot of docs together in the last years.

What do you think?

[back-2-95]

I was also thinking about docs like this. But this is really different thing than the current content of docker.md.

Maybe something like "Extending FrankenPHP" ?

[dunglas]

Or just a new section at the end: "Creating Your Own Images"?

[dunglas]

Thank you @alexander-schranz!!