Fail quickly if pre-command vars are empty

Author: piki

.buildkite/hooks/pre-command

@@ -10,3 +10,7 @@ echo "Short SHA = \"${CONTAINER_SHORT_SHA}\""
 
 export CONTAINER_BRANCH_NAME=$(echo "$BUILDKITE_BRANCH" | sed -e 's/[\/:]/_/g')
 echo "Safe branch name = \"${CONTAINER_BRANCH_NAME}\""
+
+[ -n "$CONTAINER_SEMVER" ]
+[ -n "$CONTAINER_SHORT_SHA" ]
+[ -n "$CONTAINER_BRANCH_NAME" ]