Update build-image.html.md (#4471)

* Update build-image.html.md

Without setting this field a user with get a SSH timeout error.

* Update build-image.html.md

* Update build-image.html.md

website/source/intro/getting-started/build-image.html.md

@@ -167,7 +167,11 @@ able to determine the default VPC, which the `t2` instance types require. This
 can happen if you created your AWS account before `2013-12-04`.  You can either
 change the `instance_type` to `m3.medium`, or specify a VPC. Please see
 http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/default-vpc.html for more
-information. If you specify a `vpc_id`, you will also need to set `subnet_id`.
+information. If you specify a `vpc_id`, you will also need to set `subnet_id`. 
+Unless you modify your subnet's [IPv4 public addressing attribute](
+http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-ip-addressing.html#subnet-public-ip),
+you will also need to set `associate_public_ip_address` to `true`, or set up a 
+[VPN](http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_VPN.html).
 
 ## Managing the Image