This involves a deep upgrade. Changing base distribution from debian
towards alpine, and changing the way the schedule-command relation is
handled. Consequences:
- size of image went from 195MB to 45MB
- build process simplified (only one small ``Dockerfile``)
- dependencies are not in the charm anymore but fetched on build
from our download server
- local cron business intelligence was moved in the cron charm
- extreme simplification for implementing run-once charms relation with
cron.
- some simplification for charms that adds some intelligence on their
side.
We got ``Assertion failed: bc_ctl.arg_max >= LINE_MAX (xargs.c: main: 500)`` failure
when using ``xargs``. This fix use pure builtins and avoid ``xargs`` all together.