gh-36716: Free space and repeatability for the Docker Hub workflow
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This PR implements the suggested changes of
https://github.com/sagemath/sage/pull/36385. Furthermore, it enables the
workflow to detect whether the current tag has already been pushed to
Docker Hub. Thus, if the workflow run on pushing the tag fails due to a
timeout this gives us the possibility to have a scheduler try for a
second or third time (I add a cron for Tuesday and Thursday). Such
timeouts (after 6 hours for a job) seem to happen sporadically, even
though each of the both jobs usually succeeds in less than 4 hours.
I also improve the code structure be refactoring to a reusable workflow
`docker_hub.yml`.
I tested the changes in my fork repository. To prevent from overriding
the existing tags `10.2.rc0` and `develop` I temporarily changed the tag
names to `10.2.rc0t` and `developt` for the test runs. Here is a
[successful run](https://github.com/soehms/sage/actions/runs/6814399805)
that created these tags on Docker Hub. To avoid confusion I've removed
them again. A test-run on existing tags which should skip the subsequent
steps is [this
one](https://github.com/soehms/sage/actions/runs/6828167406).
FYI: I also updated the
[README](https://hub.docker.com/r/sagemath/sagemath) on Docker Hub.
Please have a look and make corrections / additions there if necessary.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/36716
Reported by: Sebastian Oehms
Reviewer(s): Matthias Köppe, Sebastian Oehms
