Ideally, you will have a copy of
static running locally (at http://static.dev.gov.uk by default)
and that will be used to serve shared assets. This is how things will work by default if you
are running the GOV.UK development VM with foreman or bowler.
If you are running Whitehall with bundle exec rails server and don't want to
run a local copy of static, you can tell the app to use assets served
directly from the Integration environment by setting STATIC_DEV:
STATIC_DEV=https://assets-origin.integration.publishing.service.gov.uk/ bundle exec rails server
If you are only working on the Whitehall admin interface, you don't need the assets available.