make war executable

[press0]

According to these fine spring boot docker articles:

• https://stackify.com/guide-docker-java/
• https://stackify.com/spring-boot-level-up/

the war file in the spring-boot-app module should be executable but it is not:

$ java -Dspring.profiles.active=default -jar target/spring-boot-app-0.0.1-SNAPSHOT.war no main manifest attribute, in target/spring-boot-app-0.0.1-SNAPSHOT.war