diff --git a/spark/Dockerfile b/spark/Dockerfile
index c3fee78..480abf9 100644
--- a/spark/Dockerfile
+++ b/spark/Dockerfile
@@ -21,6 +21,8 @@
 
 COPY start-master.sh start-worker.sh /usr/local/bin/
 
+# these need to be separate because you can't reference prior environment
+# variables in the same ENV block
 ENV SPARK_HOME="$SPARK_INSTALL/spark" \
     SPARK_HOSTNAME="localhost" \
     SPARK_MASTER_PORT="7077" \