Enable Static proxy for development

This sets the GOVUK_PROXY_STATIC_ENABLED env var to enable the proxy to
Static in production. This is so the application continues, when Static
changes to use relative URLs for assets. See
alphagov/govuk_app_config#261 and
alphagov/govuk-puppet#11801

app.json

@@ -9,6 +9,9 @@
     "GOVUK_WEBSITE_ROOT": {
       "value": "https://www.gov.uk"
     },
+    "GOVUK_PROXY_STATIC_ENABLED": {
+      "value": "true"
+    },
     "PLEK_SERVICE_CONTENT_STORE_URI": {
       "value": "https://govuk-content-store-examples.herokuapp.com/api"
     },

startup.sh

@@ -5,6 +5,7 @@ bundle check || bundle install
 if [[ $1 == "--live" ]] ; then
   GOVUK_APP_DOMAIN=www.gov.uk \
   GOVUK_WEBSITE_ROOT=https://www.gov.uk \
+  GOVUK_PROXY_STATIC_ENABLED=true \
   PLEK_SERVICE_LICENSIFY_URI=${PLEK_SERVICE_LICENSIFY_URI-https://licensify.publishing.service.gov.uk} \
   PLEK_SERVICE_CONTENT_STORE_URI=${PLEK_SERVICE_CONTENT_STORE_URI-https://www.gov.uk/api} \
   PLEK_SERVICE_STATIC_URI=${PLEK_SERVICE_STATIC_URI-assets.publishing.service.gov.uk} \