[chore] Add cypress/factory to the list of docker images we offer. (#5130)

* add cypress/factory to the list of docker images we offer.

* Update docs/examples/docker.mdx

Co-authored-by: Matt Schile <mschile@cypress.io>

---------

Co-authored-by: Matt Schile <mschile@cypress.io>

Author: mjhenkes

docs/examples/docker.mdx

@@ -16,6 +16,11 @@ There are Docker images:
 - `cypress/browsers:<tag>` extends the base images with pre-installed browsers.
 - `cypress/included:<Cypress version>` extends the base images with
   pre-installed Cypress versions.
+- `cypress/factory:<tag>` is a docker image that can be used with docker args to
+  generate a docker container with specific versions of node, yarn, chrome,
+  firefox, edge and cypress. It is used to create the above docker images and
+  can be used by you to create a custom docker image with versions of your
+  choice.
 
 ## Examples