Run composer with dev deps in Docker

Signed-off-by: Serban Ghita <[email protected]>

docker/build.sh

@@ -2,4 +2,5 @@ echo "Start building ..."
 rm -rf vendor
 rm -f composer.lock composer.phar
 set -xe
-composer install
+# Install composer with dev dependencies so we can run tests.
+composer install --dev