Use MAILU_VERSION from Travis for Docs build

tests/build.yml

@@ -52,7 +52,10 @@ services:
 
   docs:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}docs:${MAILU_VERSION:-local}
-    build: ../docs
+    build:
+      context: ../docs
+      args:
+        version: ${MAILU_VERSION:-local}
 
   setup:
     image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}setup:${MAILU_VERSION:-local}