Fix Allowing to modify CW_INSTANCE on onbuild images

Seems using a default value is broken here, see https://github.com/moby/moby/issues/15025#issuecomment-371799311

Dockerfile.onbuild

 ARG FROM
 FROM $FROM
-ONBUILD ARG CW_INSTANCE_NAME=instance
-ONBUILD ENV CW_INSTANCE=$CW_INSTANCE_NAME
+ONBUILD ARG CW_INSTANCE_NAME
+ONBUILD ENV CW_INSTANCE=${CW_INSTANCE_NAME:-instance}
 ONBUILD USER root
 ONBUILD COPY . /src/
 ONBUILD RUN pip install -e /src