Use docker-maven-plugin for building Docker images

After porting to Spring Boot (#75) consider using `docker-maven-plugin` for building Docker images.

Related links:
- http://spring.io/guides/gs/spring-boot-docker/
- https://github.com/spotify/docker-maven-plugin
- https://github.com/rhuss/shootout-docker-maven
- https://github.com/alexec/docker-maven-plugin
