Update docker image for php8 (#8103)

diff --git a/kokoro/linux/dockerfile/test/php80/Dockerfile b/kokoro/linux/dockerfile/test/php80/Dockerfile
index d6c9b4d..8093eae 100644
--- a/kokoro/linux/dockerfile/test/php80/Dockerfile
+++ b/kokoro/linux/dockerfile/test/php80/Dockerfile
@@ -59,7 +59,7 @@
 
 # php 8.0
 RUN cd php-src \
-  && git checkout php-8.0.0alpha3 \
+  && git checkout php-8.0.0 \
   && ./buildconf --force
 RUN cd php-src \
   && ./configure \