Amend startup script to continue working
Amend startup script to continue working

I removed the STATIC_DEV and SHOW_PRODUCTION_IMAGES env var options,
this brings their functionality back for continued compatibility with
this startup script.

I also amended this to refer to production for both usages, it didn't
make much sense to me why we'd use integration static rather than the
production one.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall