Bump javadoc compilation gradle image to 6.8
diff --git a/site/docker-compose.yml b/site/docker-compose.yml
index 835c377..10db6b0 100644
--- a/site/docker-compose.yml
+++ b/site/docker-compose.yml
@@ -29,7 +29,7 @@
     volumes:
       - .:/srv/jekyll
   generate-javadoc:
-    image: gradle:6.3
+    image: gradle:6.8
     working_dir: /usr/src/calcite-avatica
     command: sh -c "gradle javadocAggregate; rm -rf site/target/avatica/javadocAggregate; mkdir -p site/target/avatica; mv build/docs/javadocAggregate site/target/avatica"
     volumes: