Add Spring Boot app to Docker Compose files

[falko]

Allow starting Camunda and the application through Docker Compose.

[falko]

There has been discussion that most the time we start the Spring Boot app using Maven for local development.

[falko]

Also there is the question how to build the image.

I'd suggest leveraging ./mvnw spring-boot:build-image (see: https://www.baeldung.com/spring-boot-docker-images#buildpacks)

[falko]

However, letting docker-compose kick off the entire Maven and Docker build also has it's convenience. And sometimes a Dockerfile is needed when some addition files should be included.