POSTGRES_PASSWORD can be more secure

[ollyg]

needs to wait for us to have our own entrypoint though, so we can call:

if [ ! -s "${PGDATA}/PG_VERSION" ]; then
  exec env POSTGRES_PASSWORD=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 1) \
    /usr/local/bin/docker-entrypoint.sh "$@"
fi