Add POSTGRES_DB_TEST variable

[pyldin601]

If this optional environment variable is set, test database will be created. Resulting name of the test database is a join of default database name and "_test" suffix.

[yosifkit]

I would say this is a duplicate of the more general #240.

#240 (comment):

Related issues: docker-library/mysql#213, docker-library/mysql#86, docker-library/mysql#18, MariaDB/mariadb-docker#63.

So, the answer is to just put scripts in /docker-entrypoint-initdb.d/:

• Allow creation of multiple databases mysql#86 (comment)
• Allow creating multiple databases on startup. mysql#18 (comment)