Add composer config to Dockerfile (#5451)

diff --git a/kokoro/linux/dockerfile/test/php/Dockerfile b/kokoro/linux/dockerfile/test/php/Dockerfile
index 8cb74c6..10862f3 100644
--- a/kokoro/linux/dockerfile/test/php/Dockerfile
+++ b/kokoro/linux/dockerfile/test/php/Dockerfile
@@ -164,3 +164,6 @@
   && chmod +x phpunit \
   && cp phpunit /usr/local/php-7.3 \
   && mv phpunit /usr/local/php-7.3-zts
+
+# Additional config
+RUN composer config -g -- disable-tls true