Update documentation

[Xiol]

The documentation here does not use the correct container image URL resulting in an error when attempting to build the container.

The syntax for the example execution-environment.yml is also incorrect:

images:
-  base_image: ghcr.io/ansible/community-ansible-dev-tools-container:latest
+  base_image:
+    name: ghcr.io/ansible/community-ansible-dev-tools:latest

[Xiol]

In fact, even that doesn't work because the build seems to be trying to use dnf whereas the container image only has microdnf. Am I supposed to be building my own container with FROM here rather than using Ansible builder? The documentation is not clear at all.