Add missing SHELL build-arg

Change-Id: Ic645c0411928f8b69c4d5b21a90462170576e7bc
diff --git a/docker-bake.hcl b/docker-bake.hcl
index fb1b0fe..14746b6 100644
--- a/docker-bake.hcl
+++ b/docker-bake.hcl
@@ -128,6 +128,7 @@
 
     args = {
         PROJECT = "nova"
+        SHELL = "/bin/bash"
     }
 
     tags = [