Update SparkJava samples

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

sparkjava-mysql/backend/Dockerfile

@@ -1,10 +1,14 @@
-FROM maven:3.5-jdk-8-alpine AS build
-COPY pom.xml .
-RUN mvn --batch-mode dependency:resolve
-COPY src/ src
+FROM maven:3.6.3-jdk-11 AS build
+WORKDIR /workdir/server
+COPY pom.xml /workdir/server/pom.xml
+RUN mvn dependency:go-offline
+
+COPY src /workdir/server/src
+
 RUN mvn --batch-mode clean compile assembly:single
 
-FROM openjdk:8-jre-alpine3.7
+FROM openjdk:11-jre-slim
+ARG DEPENDENCY=/workdir/server/target
 EXPOSE 8080
-COPY --from=build target/app.jar /app.jar
+COPY --from=build ${DEPENDENCY}/app.jar /app.jar
 CMD java -jar /app.jar