[servod] Enable udev for servo remounting

BUG=b:192605416

Change-Id: I35f735628903376053ce68aa0a96d5a86f6921ed
diff --git a/src/dockerfiles/servod/Dockerfile b/src/dockerfiles/servod/Dockerfile
index 7533204..90da1a8 100644
--- a/src/dockerfiles/servod/Dockerfile
+++ b/src/dockerfiles/servod/Dockerfile
@@ -130,4 +130,6 @@
 # container, we should not stop/start it in the case a new version is pushed.
 LABEL com.centurylinklabs.watchtower.enable="false"
 
+ENV UDEV 1
+
 COPY start_servod.sh stop_servod.sh /