feature: add Dokku deployment

[willismonroe]

I've added a back-end module for deployment to a dokku instance.

I've tested it locally and it works.

[YasharF]

Just sharing a user perspective here, not speaking as a maintainer.

One thing to consider with adding first‑class Dokku support is the long‑term maintenance cost once it's merged. From what I’ve seen, Dokku users tend to be fairly DevOps‑savvy, and many already deploy via simple SSH/git or small custom scripts.

It might be worth weighing whether this belongs as core functionality versus something documented as an example of using SSH/git for deployment or such, to avoid increasing the ongoing maintenance surface.

[willismonroe]

I agree with you in principle, certainly for me setting up a git workflow would be no issue. But this PR was inspired by my wife working on a website for herself that I'm hosting via a Dokku instance. That way I can configure it for her and it'll work when she pushes the sync button.

I could see then a client type relationship where you give someone the details for the deployment dialog so they can push to your server without having to worry about the commandline.