Prebuild webpage in Docker and load VUE_APP_CESIUM_TOKEN dynamically

[bedaberner]

This PR is based in my desire to host this app in a low ressource (k8s) environment.

Currently the prebuilt docker uses npm run dev to host the app which triggers a very ressource intense rebuild of the page.
My goal was to precompile the app during the docker build process to get around this wasteful process.
To achieve this I had to:

1. Use npm run build during the docker build process and set npm start up so it serves the precompiled page.
2. As we don't recompile on every start anymore, VUE_APP_CESIUM_TOKEN has to be loaded dinamically on startup. This is the bunch of the changes in this PR.
3. Add an error message if no VUE_APP_CESIUM_TOKEN is loaded as It took me a while to find out this was a problem.
4. Change the Cesium asset as mentioned here How to load hi res imagery? #463 (comment) to and I think this fixes How to load hi res imagery? #463.
5. Adapt the Documentation

Sorry for not splitting the changes nicely in multiple commits, I thought this would be a very easy fix but had to change more and more things