add spring-postgres application sample

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

samples/spring-postgres/backend/Dockerfile

@@ -0,0 +1,12 @@
+FROM maven:3.5-jdk-9 AS build
+COPY pom.xml .
+RUN mvn --batch-mode dependency:resolve
+COPY . .
+RUN mvn --batch-mode package
+RUN cp target/*jar target/app.jar
+
+FROM openjdk:9-jre
+EXPOSE 8080
+VOLUME /tmp
+COPY --from=build target/app.jar app.jar
+ENTRYPOINT ["java","-Djava.security.egd=file:/dev/./urandom","-jar","/app.jar"]