[TESTS] Add test container with Xdebug and allow for generation of coverage reports with 'make test'

docker/testing/coverage.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+if /var/www/social/bin/phpunit --coverage-html .test_coverage_report; then
+    exit 64
+else
+    exit 65
+fi