Update Dockerfile to use multistage building.

Dockerfile

@@ -1,10 +1,22 @@
-FROM anapsix/alpine-java:jre7
+# Multistage - Builder
+FROM maven:3.5.0-jdk-7-alpine as s3proxy-builder
 MAINTAINER Andrew Gaul <andrew@gaul.org>
 
 WORKDIR /opt/s3proxy
+COPY . /opt/s3proxy/
+
+RUN mvn package
+
+# Multistage - Image
+FROM java:7u121-jre-alpine
+MAINTAINER Andrew Gaul <andrew@gaul.org>
+
+WORKDIR /opt/s3proxy
+
 COPY \
-    target/s3proxy \
-    src/main/resources/run-docker-container.sh \
+    --from=s3proxy-builder \
+    /opt/s3proxy/target/s3proxy \
+    /opt/s3proxy/src/main/resources/run-docker-container.sh \
     /opt/s3proxy/
 
 ENV \