use mkdocs:testing image

This is local and contains an updated mkdocs that supports more types of
validations.

docker-compose.yml

@@ -21,7 +21,7 @@ services:
     ports:
       - 5432:5432
   mkdocs:
-    image: hackforlaops/mkdocs:latest
+    image: hackforlaops/mkdocs:testing
     command: mkdocs serve --dev-addr 0.0.0.0:8000
     ports:
       - 8005:8000