Development: only pull the images required

We are not using `readthedocs/build:latest` anymore for the clone step. We are
now using `readthedocs/build:ubuntu-20.04` for it, so that image is the
required one.

Author: humitos

docs/dev/install.rst

@@ -54,7 +54,7 @@ Set up your environment
 
    .. prompt:: bash
 
-      inv docker.pull --only-latest
+      inv docker.pull --only-required
 
 #. start all the containers: