Add mechanism for custom templates

[fliphess]

Hey @basnijholt

I'm splitting the template feature it in two parts: First only the mechanism to make custom templates possible, leaving the templates directory for now as an empty dir, and in the other PR just the templates in a clean and tidy state.

I need some more time to get the html templates theirselves working in an acceptable state, but I want to avoid the other MR getting way to big, I think it's already a lot without all the html templates.

So here stage 1:

• A mechanism to allow custom templates to be used
• Documentation for this feature
• Some docker optimization that only copies the created binary from another layer, leaving all the build stuff behind to save some space and use the layered cache a bit more.

(I've removed the local devcontainer that I initially added by accident, left in because I thought it was useful, but now removed again as it's another 6 pages of text to review in a single PR: We can always add it later)

[basnijholt]

Once again some great work! 🎉

Ready to merge when addressing my small comment.

[fliphess]

Thanks! Added :) I will merge this one, rebase and continue to the next :) 👍