Enable Review Apps

Procfile

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+web: GOVUK_ASSET_ROOT=https://${HEROKU_APP_NAME}.herokuapp.com bin/rails server -p $PORT -e $RAILS_ENV

app.json

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+{
+  "name": "Government Frontend",
+  "repository": "https://github.com/alphagov/government-frontend",
+  "env": {
+    "GOVUK_APP_DOMAIN": {
+      "value": "www.gov.uk"
+    },
+    "GOVUK_ASSET_ROOT": {
+      "value": "https://government-frontend.herokuapp.com"
+    },
+    "GOVUK_WEBSITE_ROOT": {
+      "value": "https://www.gov.uk"
+    },
+    "PLEK_SERVICE_CONTENT_STORE_URI": {
+      "value": "https://www.gov.uk/api"
+    },
+    "PLEK_SERVICE_STATIC_URI": {
+      "value": "assets.digital.cabinet-office.gov.uk"
+    },
+    "RAILS_SERVE_STATIC_ASSETS": {
+      "value": "yes"
+    },
+    "SECRET_KEY_BASE": {
+      "generator": "secret"
+    },
+    "HEROKU_APP_NAME": {
+      "required": true
+    }
+  },
+  "image": "heroku/ruby",
+  "buildpacks": [
+    {
+      "url": "https://github.com/heroku/heroku-buildpack-ruby"
+    }
+  ],
+  "addons": []
+}