Container initialization fails when POSTGRES_USER != postgres

[azapasnik]

Container initialization fails with error in case when environment variable POSTGRES_USER is set to something that doesn't equals to postgres.

Initialization error:

...
/usr/local/bin/docker-entrypoint.sh: running /docker-entrypoint-initdb.d/routing.sh
2019-04-23 13:27:42.903 UTC [96] FATAL:  role "postgres" does not exist

To reproduce:
docker run -e POSTGRES_USER=test -e POSTGRES_DB=test -e POSTGRES_PASSWORD=test starefossen/pgrouting