format: format the invoker's Dockerfile

Author: Zinuo

core/invoker/Dockerfile

@@ -18,8 +18,8 @@
 FROM scala
 
 ENV UID=1001 \
-    NOT_ROOT_USER=owuser
-    ENV DOCKER_VERSION=18.06.3-ce
+    NOT_ROOT_USER=owuser \
+    DOCKER_VERSION=18.06.3-ce
 # If you change the docker version here, it has implications on invoker runc support.
 # Docker server version and the invoker docker version must be the same to enable runc usage.
 # If this cannot be guaranteed, set `invoker_use_runc: false` in the ansible env.