fix docker build on btrfs (spring-petclinic#255)

docker/Dockerfile

@@ -14,7 +14,15 @@ EXPOSE ${EXPOSED_PORT}
 ENV SPRING_PROFILES_ACTIVE docker
 
 COPY --from=builder application/dependencies/ ./
+
+# fix for https://stackoverflow.com/questions/51115856/docker-failed-to-export-image-failed-to-create-image-failed-to-get-layer
+# (only last copy caused issue)
+# this seems to be triggered by using btrfs:
+# https://github.com/moby/moby/issues/36573
+RUN true
 COPY --from=builder application/spring-boot-loader/ ./
+RUN true
 COPY --from=builder application/snapshot-dependencies/ ./
+RUN true
 COPY --from=builder application/application/ ./
 ENTRYPOINT ["java", "org.springframework.boot.loader.launch.JarLauncher"]