add dockerfile to build java7 version of the plugin which will be shared

Dockerfile

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+FROM java:7
+MAINTAINER Jan Burkhardt <[email protected]>
+RUN useradd -d /home/theuser -m theuser
+USER theuser
+VOLUME /dist
+WORKDIR /dist
+RUN java -version

dist

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+#!/bin/sh
+RANDOM_TAG=jsr305_plugin_dist_$(tr -dc "a-z" < /dev/urandom | head -c 8)
+
+docker build -t $RANDOM_TAG .
+docker run --rm -v $PWD:/dist $RANDOM_TAG sh -c "./gradlew clean jar"
+docker rmi --no-prune=true $RANDOM_TAG