custom docs_dir and custom domain

[frealmyr]

The CUSTOM_DOMAIN environment variable does not work when also using a custom docs_dir in mkdocs.yml.

When specifying a custom docs_dir such as

docs_dir: articles/

Build will fail with the following error:

/action.sh: line 24: /github/workspace/docs/CNAME: No such file or directory

The correct folder to target for CNAME file creation should in this instance should be articles/.

A neat way to solve this would be to lookup the key in mkdocs.yml (using yq maybe?), and the easiest would be to add another environment variable.

[mhausenblas]

Wanna PR it? :)