Dockerfile: Add missing r-base-core package for lister (cran)

Without this, we cannot run the cran lister.

Depends on !101 (closed) (-> to make the scheduler-runner actually schedule)

Test Plan

# rebuild container
$ doco swh-lister up -d
$ workon swh
$ SCHEDULER_API_URL=http://localhost:5008/; swh scheduler --url $SCHEDULER_API_URL task add list-cran

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Migrated from D2061 (view on Phabricator)