Clean up docker image and update dockerignore

[lukasrad02]

The current docker build process is inefficient and cannot be cached properly: We add the full working directory to the build container and the .dockerignore file is not complete. This leads to full rebuilds (npm run deployment) if unrelated files are changed, for example the nginx config.

We should only COPY the necessary files into the container and check whether files aren't needed by any of the stages (for example the Dockerfile itself) to add them to the .dockerignore file.