Use s6-overlay mechanisms for cron environment

[MarcelWaldvogel]

While trying to understand the s6-overlay and https-portal workings more in depth for #261, I noticed that s6-overlay provides with-contenv. Using that obviates the need for maintaining the environment in /etc/cron_env.sh a second time. This PR makes things more s6-overlay-ish (and simplifies operation). This addresses question 1 from #260.

• logrotate and its children do not require the environment
• with-contenv loads the container environment for /bin/renew
  • Could also be done in the #! line of renew, but keeping it here reminds other cron users of the non-automatic environment
  • Unrelated: Also limit excessive sleep precision to milliseconds

(When reading the s6-overlay documentation, I got the impression that just setting S6_KEEP_ENV globally (i.e., from the Dockerfile) would make the Dockerfile environment available to all processes. However, this does not seem to be the case, it just suppressed reloading the environment in child with-contenv-based processes.)

[SteveLTN]

I will try it. I made the mistake before, trusting some documentation and cron jobs ended up not having the ENVs. One need to actually see the job running from cron (not manually trigger it in bash) to confirm.

[MarcelWaldvogel]

Yes, documentation can indeed be misleading.

I am running this version on the two servers I mentioned previously and have

1. created a debug entry, which would ran env or with-contenv env every minute, and without with-contenv, it just prints HOME, LOGNAME, PATH, SHELL, and PWD; with with-contenv, it prints DOMAINS etc. as well.
2. changed the crontab firing time for renew and logrotate. I got output for all the renewal deferrals. It did not rotate logs, however (see another issue).