Allow additional location for start_script.sh

The entrypoint.sh doesn't start a server per default, but requires configuration of what to do via start_script.sh, which isn't present in the Docker image, but has to be provided by the user.

Some container solutions (e.g. QNAP Container Station) don't allow mounting files into containers, i.e. only full folders can be mounted, so adding the start_script.sh without changing the image isn't possible.

This PR allows the start_script.sh to be provided in the config folder, which often (usually?) is mounted from the host already. Mine simply contains "npm run server".