update env variable to accept injection

Author: ohdoking

src/main/resources/application.properties

@@ -1,4 +1,4 @@
-spring.datasource.url=jdbc:postgresql://localhost:5432/hello_db
-spring.datasource.username=postgres
-spring.datasource.password=postgres
+spring.datasource.url=${DB_URL}
+spring.datasource.username=${DB_USERNAME}
+spring.datasource.password=${DB_PASSWORD}
 spring.jpa.hibernate.ddl-auto=update