Merge pull request #4837 from circleci/BytesGuy-patch-2

Clarify docker installation in 3rd party images

jekyll/_cci2/building-docker-images.md

@@ -94,7 +94,7 @@ jobs:
           docker push CircleCI-Public/circleci-demo-docker:$TAG
 {% endhighlight %}
 
-If the primary container you are using doesn't already have the docker CLI installed, then [you will need to install it](https://docs.docker.com/install/#supported-platforms).
+**Note:** The [CircleCI convenience images](https://circleci.com/docs/2.0/circleci-images/) for the Docker executor come with the Docker CLI pre-installed. If you are using a third-party image for your primary container that doesn't already have the Docker CLI installed, then [you will need to install it](https://docs.docker.com/install/#supported-platforms) as part of your job before calling any `docker` commands.
 
 ```
       # Install via apk on alpine based images