Move to S6

[modem7]

This is a WIP PR to move us to S6 and simplify + make the image more resilient.

• Added S6 (which should also fix https://github.com/borgmatic-collective/docker-borgmatic/pull/173)
• Added Docker env var to allow for docker operations within the container
• Added Cron var for additional cron jobs
• Removed dedicated Apprise package as that is now part of the Borgmatic package.
• Removed Supercronic in favour of basic cron
• Entrypoint moved to /root/etc/s6-overlay/s6-rc.d/svc-cron/run (This needs reviewing the most as there has been additional change/reversions that may need resolving/modifying).

Environment Variables

Variable	Description	Possible Values	Default
DOCKERCLI	Install DockerCLI	true	Empty
CRON	Cron times	cron time, disabled	0 1 * * *
CRON_COMMAND	Command cron will run	borgmatic --stats -v 0 2>&1	borgmatic --stats -v 0 2>&1
EXTRA_CRON	Extra cron lines	0 5 2 * * command1	Empty

• @Psycho0verload please could you review the changes that will be required to cover the changes that were done in #275. I haven't had time to implemented them yet, and I don't want to introduce a regression.

Once merged, this PR will close #233 #242 #272 #301