docker: Explicit the scheduler runner for save-code-now

Related to swh/infra/sysadm-environment#3367 (closed)

Depends on swh-scheduler!236 (closed) (and the override in the docker-compose.override.yml for swh-scheduler-runner and swh-scheduler-runner-priority)

Test Plan

doco up -d Then play with the save code now and ensure the priority tasks are indeed prioritize with the right new scheduler (it is):

swh-scheduler-runner-priority_1  | wait-for-it: amqp:5672 is available after 0 seconds
swh-scheduler-runner-priority_1  | INFO:swh.scheduler.celery_backend.runner:Grabbed 1 tasks load-git (priority)
swh-scheduler-runner-priority_1  | INFO:swh.scheduler.cli.admin.runner:Scheduled 1 tasks

The other ones don't bulge unless you start listing stuff:

swh-scheduler-runner_1           |  postgresql is up!
swh-scheduler-runner_1           | (1 row)
swh-scheduler-runner_1           |
swh-scheduler-runner_1           | wait-for-it: waiting for swh-scheduler:5008 without a timeout
swh-scheduler-runner_1           | wait-for-it: swh-scheduler:5008 is available after 0 seconds
swh-scheduler-runner_1           | Starting the swh-scheduler start-runner
swh-scheduler-runner_1           | wait-for-it: waiting for amqp:5672 without a timeout
swh-scheduler-runner_1           | wait-for-it: amqp:5672 is available after 0 seconds
swh-scheduler-runner_1           | INFO:swh.scheduler.celery_backend.runner:Grabbed 1 tasks index-origin-metadata
swh-scheduler-runner_1           | INFO:swh.scheduler.cli.admin.runner:Scheduled 1 tasks
...

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Migrated from D5851 (view on Phabricator)