create setup_composer script

docker/php/scripts/setup_composer.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+setup_composer() {
+  echo "Installing dependencies..."
+
+  mkdir -p /var/www/.composer/cache
+  chown -R www-data:www-data /var/www/.composer
+
+  if [ -f composer.json ]; then
+    run composer install --no-interaction --prefer-dist
+  fi
+}