Add dockerfile for compatibility tests (#1024)

Author: piotrpio

Dockerfile.compat

@@ -0,0 +1,9 @@
+FROM gradle:6-jdk11 AS build
+COPY --chown=gradle:gradle . /home/gradle/src
+WORKDIR /home/gradle/src
+RUN gradle clean compileTestJava --no-daemon
+
+FROM openjdk:11-jre-slim
+RUN mkdir /app
+COPY --from=build /home/gradle/src/build/classes/java /app
+ENTRYPOINT ["java", "--class-path=/app/main:/app/test", "io.nats.compatibility.ClientCompatibilityMain"]