Allow to run some shell scripts when starting the Nuxeo Docker image

Similarly to what exists in the nuxeo official Docker image:

Launching custom shell scripts
You can add your own shell scripts in a special /docker-entrypoint-initnuxeo.d directory. When ending in .sh, they will be run on default entrypoint startup.

 See https://github.com/nuxeo/docker-nuxeo/blob/master/master/docker-entrypoint.sh#L47.