Add link to docker.md

docs/GETTING_STARTED.md

@@ -3,6 +3,9 @@
 Demonstrates setting up your own PKI and certificate authority using `step ca`
 and getting certificates using the `step` command line tool and SDK.
 
+Check out [Getting started with docker](docker.md) to run step certificates
+using docker.
+
 ![Animated terminal showing step ca init in practice](https://smallstep.com/images/blog/2018-12-04-unfurl.gif)
 
 ## Prerequisites