ci: Downgrade docs docker image Ubuntu version to focal

The currently used version, impish, is not updatable anymore since July
the 14th and we don't have a newer version in our repository. Downgrade
this to focal to fix the container image builds.

Fixes: eclipse/oniro-core/oniro#724


Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>