Remove var/www/html in collector

[eloiferrer]

Issue description: In the collector stage we should remove the var/www/html directory before we start copying the extensions. Otherwise we might create incompatibility issues between the lts mediawiki version and the cloned branch.

TODOS:

• [ ] RUN rm -r /var/www/html/* in the collector stage
• [ ] make sure the resulting image works

Acceptance-Criteria

Checklist for this issue:

• [ ] Assignee has been set for this issue
• [ ] All fields of the issue have been filled
• [ ] Issue has been assigned to the main project
• [ ] Code was merged
• [ ] Feature branch has been deleted and issues were updated / closed
• [ ] Issue is tracked by an epic, or the label 'non-epic' is set to the issue.

[eloiferrer]

This has been now implemented: https://github.com/MaRDI4NFDI/docker-wikibase/commit/17463992e26c109f1b1b6f4edbff78e6be24c80f