c45094c broke the workflow: CI triggered on PR (modified code) was
actually testing master codebase (untouched code). Solution:
- Drop pull_request_target: use pull_request
- Drop all docker jobs (which need pull_request_target to be based
on master to be able to access opencollab secrets)
- Remove opencollab secrets (docker login and password)
- Drop coverage jobs (OK with TravisCI but never worked with
github-actions)