Cleanup / performance improvement for docs docker build

Note that this won't improve performance at this stage as the docs:latest image doesn't contain the layers that should be cached yet. After merging we should verify how much faster it is.

Features

• Remove the second build stage (where the version.json file was generated), generate it before building the image instead
• Remove the HAVEN_VERSION build argument as it's no longer required
• Use COPY --link in the final build stage
• Run npm install before npm build so it can be cached
• Enable buildkit and setup inline caching

Testing

• Check if version.json is still generated correctly
• Verify if every page renders properly