Explicitly download 'node:7.9.0' docker image so the jenkins job is
fixed.

Change-Id: I86cb091e045fe8adb3d9cbf883770aafee1dfcf8
diff --git a/docker_images.yml b/docker_images.yml
index 07168df..6ebdb26 100644
--- a/docker_images.yml
+++ b/docker_images.yml
@@ -34,6 +34,7 @@
   - "nginx:1.13"
   - "onosproject/onos:1.10.3"
   - "redis:3.2"
+  - "node:7.9.0" # remove once the issue with node:candidate is resolved
 
 # buildable_images - list of images that we build from Dockerfiles
 #