Align dockerization process with spring.io guides

(And with existing Spring Cloud projects.) Uses a maven plugin to
build the image. It can be pushed manually, or else using
-Ddocker.image.prefix=<path_to_repo>. Removes some of the hard-coded
host names and stuff that came with the old version.

spring-cloud-stream-samples/docker-compose.yml

@@ -0,0 +1,4 @@
+redis:
+  image: redis
+  ports:
+    - "6379:6379"