Merge branch 'patch-3' into 'master'

Document how to define default image to run using Kubernetes executor

See merge request !668

docs/install/kubernetes.md

@@ -23,8 +23,12 @@ data:
       executor = "kubernetes"
       [runners.kubernetes]
         namespace = "gitlab"
+        image = "busybox"
 ```
 
+Where `image` (optional) is the default Docker image to run jobs on top of.
+
+
 Then create a `Deployment` or `ReplicationController` which uses the `ConfigMap`.
 This is an example of a `Deployment`: