Add missing SHELL build-arg

Change-Id: Ic645c0411928f8b69c4d5b21a90462170576e7bc
diff --git a/.gitignore b/.gitignore
index 2ad4866..8aca44a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,6 +25,7 @@
 collections/
 *.egg-info
 CHANGELOG.rst
+target
 
 ## Editors
 .idea