Add support for docker-compose

I'm somewhat more confident running an unknown docker container than to install unknown software on my mashine, and to test some simple changes, docker would be a great addition.

I tried to sneak in a docker-compose file with #284 but @altinnadmin wanted to wait and see if we could fix some minor issues first.

* Not official image
* Auto update of rendered pages does not work (at least on windows when not using the WSL2 filesystem.)

```docker-compose
version: "3.8"
services:
server:
image: klakegg/hugo:0.83.1
command: server
volumes:
- ".:/src"
ports:
- "1313:1313"
```

Review issue #284 and the proposed docker-compose configuration first, then check the repository's local documentation workflow. Done means the documented compose setup starts the Hugo server reliably, addresses the noted image concern, and supports rendered-page updates where the project expects them.

Written by the indexing model from the issue text.