Use the container to populate the pages content

The pages job served only (it seems) to upload the output of htmlcov on the master branch, with no expiry time. It seems fine to me to upload the artefacts as part of the container job. Since these artefacts are not very interesting for historical analysis, get rid of them after a day as well.

Removing the containers is not strictly necessary, but why not. The trick to make the artefact upload work again is to bind-mount the output directory of coverage.