resources: update HACC README

Update HACC's README to provide additional details about why it
recommends using a new docker image for it.

Change-Id: Ib0801e49db5fbab031819a8540fd075ac20e7ab8
Reviewed-by: Bobby R. Bruce <>
Maintainer: Bobby R. Bruce <>
Tested-by: Bobby R. Bruce <>
diff --git a/src/gpu/halo-finder/ b/src/gpu/halo-finder/
index cbfb685..9bf123c 100644
--- a/src/gpu/halo-finder/
+++ b/src/gpu/halo-finder/
@@ -33,6 +33,9 @@
 In order to test the GPU code in halo-finder, we compile and run ForceTreeTest.
 To build the Docker image and the benchmark:
+Note: HACC requires a number of environment variables to be set to compile and run correctly.  Our Dockerfile sets these flags appropriately for you.  This Dockerfile automatically runs when a new docker image is created, which is why our instructions below recommend doing this.  If you would prefer not doing this, then you will need to pass in these environment variables using -e.
 cd src/gpu/halo-finder
 docker build -t <image_name> .