Move all samples to the root dir

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

sparkjava-mysql/backend/Dockerfile

@@ -0,0 +1,10 @@
+FROM maven:3.5-jdk-8-alpine AS build
+COPY pom.xml .
+RUN mvn --batch-mode dependency:resolve
+COPY src/ src
+RUN mvn --batch-mode clean compile assembly:single
+
+FROM openjdk:8-jre-alpine3.7
+EXPOSE 8080
+COPY --from=build target/app.jar /app.jar
+CMD java -jar /app.jar