Push maven-index-exporter image to docker hub

We want to:

• make the ci build the image (through [1] which sounds sensible enough)
• out of a tag (like we do the other modules either swh or swh-dependency package)
• out of a commit on master (tests ok then push with tag "latest")
• Make the job declaratively installed out of the swh-jenkins-jobs declarations

Requisite:

• Figure out how to create a swh-ci account and register it to the organization (email: swh-ci@softwareheritage.org; login: swh-ci, ...). For now we stuck with one user of the organization since there is quite some limitations on that number (3).

• [1] https://www.jenkins.io/doc/book/pipeline/docker/#building-containers swh organization on dockerhub: softwareheritage inspiration: https://forge.softwareheritage.org/source/swh-docker/browse/master/images/build_images.sh

Note: To be able to login, one user should be part of the organization.

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Migrated from T4105 (view on Phabricator)