Support executing scripts inside the `/docker-entrypoint.d` directory

[Fabccc]

When I wanted to add SMTP configuration based on environment variable, I've been stuck because I couldn't execute script before redmine started. My workaround was to create a custom Docker image based on this repo, and replace with sed the last line of the docker-entrypoint.sh to include executing scripts of directory inside of docker-entrypoint.d. I think it's a common practice to be able to add user's defined script here.

I will provide a PR for this functionnality.