refactor: move the renewal loop into a script, reset the base entrypoint

The loop lived in a shell-form ENTRYPOINT one-liner: unlintable, uncommentable,
and expanded by docker into two nested shells. It is a file now, and the loop
is CMD with the base image's `certbot` entrypoint reset — CMD is appended to
ENTRYPOINT rather than replacing it, so without the reset the loop would have
arrived as arguments to certbot. In exchange a one-off run replaces the loop
outright and needs no --entrypoint:

    docker run --rm -v letsencrypt:/etc/letsencrypt <image> certbot certificates

`wait $(jobs -p)` became a bare `wait`: the command substitution runs in a
subshell that reports the parent's jobs in bash but not in dash, and bare
`wait` waits for every background job in either. A failed pass is now reported
rather than passing silently, and the trap covers INT as well so Ctrl-C in an
interactive run works.

What this does not fix: a signal arriving while certbot is talking to Let's
Encrypt is held until that call returns, because a POSIX shell runs a trap only
after the foreground command finishes. That can outlast docker's ten-second
stop grace. Set stop_grace_period on the service if a clean stop matters.

docker/Dockerfile

@@ -12,5 +12,12 @@ RUN chmod 755 /opt/scripts/*.sh
 # Expose port 380
 EXPOSE 380
 
-ENTRYPOINT /bin/sh -c 'trap exit TERM; while :; do /opt/scripts/1-renew-cert.sh ; sleep 12h & wait $(jobs -p); done;'
+# The base image runs certbot itself (ENTRYPOINT ["certbot"]). Reset it, or the
+# CMD below is appended as arguments to certbot instead of replacing it. Both
+# directives are in exec form: a shell-form ENTRYPOINT makes docker drop CMD
+# altogether, and a shell-form CMD would expand into a nested /bin/sh -c.
+# Keeping the loop in CMD is what lets a one-off run replace it outright:
+#   docker run --rm -v letsencrypt:/etc/letsencrypt <image> certbot certificates
+ENTRYPOINT []
+CMD ["/opt/scripts/entrypoint.sh"]