Revert "Build the docs during tests"

This reverts commit b287a85124.

tests/build.yml

@@ -45,9 +45,3 @@ services:
   none:
     image: mailu/none:$VERSION
     build: ../core/none
-
-  docs:
-    image: mailu/docs:$VERSION
-    build:
-      context: ../
-      dockerfile: ../docs/Dockerfile