Ensure posthtml loads env variables

parcel-bundler/parcel#1209 (comment)

posthtml.config.js

@@ -1,3 +1,27 @@
+// https://github.com/parcel-bundler/parcel/issues/1209#issuecomment-942927265
+const dotenv = require('dotenv');
+
+const NODE_ENV = process.env.NODE_ENV;
+
+const dotenvFiles = [
+  '.env',
+  // Don't include `.env.local` for `test` environment
+  // since normally you expect tests to produce the same
+  // results for everyone
+  NODE_ENV === 'test' ? null : '.env.local',
+  `.env.${NODE_ENV}`,
+  `.env.${NODE_ENV}.local`
+].filter(Boolean);
+
+const env = {};
+
+for (let dotenvFile of dotenvFiles) {
+  const config = dotenv.config({ path: dotenvFile });
+  if (config.parsed) {
+    Object.assign(env, config.parsed);
+  }
+}
+
 module.exports = {
   plugins: {
     'posthtml-expressions': {