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/scripts/entrypoint.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# The renewal loop: run a pass, sleep 12 hours, repeat. PID 1 of the container.
+#
+# Deliberately not `set -e`: a pass that fails — Let's Encrypt unreachable,
+# HAProxy refusing the certificate — must not end the loop, it must be retried
+# on the next one.
+#
+# The trap is not decoration. PID 1 has no default signal dispositions, so a
+# signal with no handler installed is ignored outright and `docker stop` would
+# always have to fall through to SIGKILL.
+#
+# `sleep &` followed by `wait`, rather than a plain `sleep`: a foreground
+# command keeps the shell from running a trap until it finishes, so a plain
+# sleep would leave the container deaf to SIGTERM for up to 12 hours. `wait` is
+# interruptible. Bare `wait` rather than `wait $(jobs -p)` — 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.
+#
+# One thing this cannot fix: a signal arriving while 1-renew-cert.sh is in the
+# foreground is deferred until that script returns, and an issuance talking to
+# Let's Encrypt can outlast docker's 10-second stop grace. Give the service a
+# longer `stop_grace_period` if that matters.
+
+trap 'exit 0' TERM INT
+
+while :; do
+  /opt/scripts/1-renew-cert.sh \
+    || echo "entrypoint: renewal pass failed, retrying in 12h" >&2
+  sleep 12h &
+  wait
+done