diff --git a/README.md b/README.md
index cbad97b2b33e68214266674f1f051f933157d617..07c5f28104bd4118166799c031366f6289b75674 100644
--- a/README.md
+++ b/README.md
@@ -13,7 +13,7 @@ This is the MIP backend.
 
 ## Usage in building environment
 
-* Build a versioned image: `./captain_build.sh` or `captain build`
+* Build a versioned image: `./captain_build.sh` (only if `$WORKSPACE=$(pwd)`) or `export WORKSPACE=<path-to-workspace> && captain build`
 * Build and test an image: `./captain_test.sh` or `captain test`
 * Run the latest image with a non-persistent postgres (only use this for testing purpose): `./captain_run.sh`