Add using of PGPASSWORD for psql

Given example is not complete at least for official postgres image. I have added setting of psql password over POSTGRES_PASSWORD env variable.

compose/startup-order.md

@@ -63,7 +63,7 @@ script:
         shift
         cmd="$@"
 
-        until psql -h "$host" -U "postgres" -c '\q'; do
+        until PGPASSWORD=$POSTGRES_PASSWORD psql -h "$host" -U "postgres" -c '\q'; do
           >&2 echo "Postgres is unavailable - sleeping"
           sleep 1
         done