Issue #112 Allow running container under different user to match jenkins home volumes

Put copy_reference_file.log so container starts with different user id
diff --git a/Dockerfile b/Dockerfile
index d5b2da5..4b8f4fb 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -39,8 +39,7 @@
 # will be used by attached slave agents:
 EXPOSE 50000
 
-ENV COPY_REFERENCE_FILE_LOG /var/log/copy_reference_file.log
-RUN touch $COPY_REFERENCE_FILE_LOG && chown jenkins.jenkins $COPY_REFERENCE_FILE_LOG
+ENV COPY_REFERENCE_FILE_LOG $JENKINS_HOME/copy_reference_file.log
 
 USER jenkins