diff --git a/test/docker/README.md b/test/docker/README.md
new file mode 100644
index 00000000..bc41a717
--- /dev/null
+++ b/test/docker/README.md
@@ -0,0 +1,5 @@
+# Manually build the image
+
+```bash
+docker build --tag odr_core_test test/docker
+```
diff --git a/test/docker/compare_output_server.sh b/test/docker/compare_output_server.sh
index bb08cd78..57b2f2b0 100755
--- a/test/docker/compare_output_server.sh
+++ b/test/docker/compare_output_server.sh
@@ -4,8 +4,14 @@ REF="test/data/reference-output/"
 OBS="cmake-build-relwithdebinfo/test/output/"
 DRIVER="firefox"
 
-# manually build the image
-#docker build --tag odr_core_test test/docker
+if [ ! -d "$REF" ]; then
+  echo "Reference output directory does not exist: $REF"
+  exit 1
+fi
+if [ ! -d "$OBS" ]; then
+  echo "Observed output directory does not exist: $OBS"
+  exit 1
+fi
 
 docker run -ti \
   -v $(pwd):/repo \