made some config available via env vars

frontend/gatsby-config.js

@@ -20,15 +20,15 @@ module.exports = {
     {
       resolve: 'gatsby-plugin-matomo',
       options: {
-        siteId: '53',
-        matomoUrl: 'https://stats.waag.org/',
+        siteId: process.env.MATOMO_ID || '53',
+        matomoUrl: process.env.MATOMO_URL || 'https://stats.waag.org/',
         siteUrl: 'https://prototype.openschoolingnavigator.eu/'
       }
     },
     {
       resolve: `gatsby-source-drupal`,
       options: {
-        baseUrl: `https://openschoolingnavigator.eu/`,
+        baseUrl: process.env.CMS_URL || `https://openschoolingnavigator.eu/`,
         skipFileDownloads: true,
       },
     },