Add script and README

Author: mrts

README.md

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+# Using multiple databases with official PostgreSQL Docker image
+
+The [official recommendation](https://hub.docker.com/_/postgres/) for creating
+multiple databases is as follows:
+
+*If you would like to do additional initialization in an image derived from
+this one, add one or more `*.sql`, `*.sql.gz`, or `*.sh` scripts under
+`/docker-entrypoint-initdb.d` (creating the directory if necessary). After the
+entrypoint calls `initdb` to create the default `postgres` user and database,
+it will run any `*.sql` files and source any `*.sh` scripts found in that
+directory to do further initialization before starting the service.*
+
+This directory contains a script to create multiple databases using that
+mechanism.
+
+## Usage
+
+Clone the repository, mount it's directory into the official Docker image and
+declare database names separated by commas in `POSTGRES_MULTIPLE_DATABASES`
+environment variable as follows (`docker-compose` syntax):
+
+    myapp-postgresql:
+        image: postgres:9.6.2
+        volumes:
+            - ../docker-posgresql-multiple-databases:/docker-entrypoint-initdb.d
+        environment:
+            - POSTGRES_MULTIPLE_DATABASES=db1,db2
+            - POSTGRES_USER=myapp
+            - POSTGRES_PASSWORD=

create-multiple-postgresql-databases.sh

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+#!/bin/bash
+
+set -e
+set -u
+
+function create_user_and_database() {
+	local database=$1
+	echo "  Creating user and database '$database'"
+	psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" <<-EOSQL
+	    CREATE USER $database;
+	    CREATE DATABASE $database;
+	    GRANT ALL PRIVILEGES ON DATABASE $database TO $database;
+	EOSQL
+}
+
+if [ $POSTGRES_MULTIPLE_DATABASES ]; then
+	echo "Multiple database creation requested: $POSTGRES_MULTIPLE_DATABASES"
+	for db in $(echo $POSTGRES_MULTIPLE_DATABASES | tr ',' ' '); do
+		create_user_and_database $db
+	done
+	echo "Multiple databases created"
+fi