doc: Add a "how to test your own loader in docker" section

one exists for package loader [1].

We should have a dedicated section on how to run a new dvcs loader somewhere in our documentation.

Either add a section to the existing vcs loader overview [2]. Or create a full new howto.

This should detail how to mount local overrides within the least required amount of services (so that even save code now works with the new type for example).

• [1] https://docs.softwareheritage.org/devel/swh-loader-core/package-loader-tutorial.html?highlight=run#running-your-loader

• [2] https://docs.softwareheritage.org/devel/swh-loader-core/vcs-loader-overview.html

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Migrated from T3628 (view on Phabricator)