The PHP DOM extension is required

I'm getting the following error when running a test scenario on a minimal Ubuntu Bionic image:

```
Fatal error: Class 'DOMDocument' not found (Behat\Testwork\Call\Exception\FatalThrowableError)
```

It is thrown in `Behat\Testwork\Output\Printer\JUnitOutputPrinter`.

composer.json

@@ -15,6 +15,7 @@
 
     "require": {
         "php": ">=5.3.3",
+        "ext-dom": "*",
         "ext-mbstring": "*",
         "behat/gherkin": "^4.6.0",
         "behat/transliterator": "^1.2",