Make the image prepare its own data directory (closes #262)

A host directory mounted at /var/lib/neoirc kept the host's owner, so
neoircd could not create state.db. The image now starts as root in
deploy/docker-entrypoint.sh, which creates /var/lib/neoirc, gives it to
neoirc with mode 0700, and runs neoircd as neoirc through su-exec.

Model: opus-5-5

deploy/docker-entrypoint.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+# deploy/docker-entrypoint.sh: the image's ENTRYPOINT, run as root. A host
+# directory mounted at /var/lib/neoirc keeps the host's owner, so this gives
+# it to neoirc before running neoircd as neoirc.
+set -eu
+
+mkdir -p /var/lib/neoirc
+chown -R neoirc:neoirc /var/lib/neoirc
+chmod 0700 /var/lib/neoirc
+exec su-exec neoirc neoircd "$@"