Add "DOCKER_INITDB_EXTRA_SQL" environment variable - #947

We try to add as few "extra" features as possible and I think this one isn't really necessary. We already have the ability to run arbitrary SQL and shell scripts on first initialization. I think this would set a precedent for similar requests in MySQL and friends. I would be open to POSTGRES_SUPERUSER (#175 (comment)), since creating a root/super user along with a limited user already exists for MySQL and friends.

The difference in the docker run lines don't seem that drastic. If users are in an environment where bind-mounts, config maps, or custom images are difficult or not allowed, then a simple command change is enough to have the same functionality. I'm fairly certain that all our push back in other images to force command argument customization and not just environment variables, means that they should all support this.

$ # with proposed change
$ docker run -d -e DOCKER_INITDB_EXTRA_SQL="really complex sql; things;" -e ... mysql:8

$ # without proposed change
$ docker run -d -e EXTRA_SQL='really complex sql; things;' -e ... mysql:8 sh -c 'echo "$EXTRA_SQL" > /docker-entrypoint-initdb.d/00-custom.sql && exec docker-entrypoint.sh mysql'
$ # could even skip the environment variable altogether
$ docker run -d -e ... mysql:8 sh -c 'echo "really complex sql; things;" > /docker-entrypoint-initdb.d/00-custom.sql && exec docker-entrypoint.sh mysql'