Container sets its data directory's owner and mode itself (#353)

Closes #340.

The image no longer sets `USER`. Its new `ENTRYPOINT`, `deploy/docker-entrypoint.sh`, starts as root, creates `DATA_DIR` if missing, gives the directory and anything in it owned by another user to `webhooker` (UID 1000), sets the directory to `0750`, and runs the command as `webhooker` through `su-exec`. An empty root-owned bind mount, or data left by another UID, now works as mounted; the app never runs as root and is still PID 1. `CMD` is still `/app/webhooker`, so the `resetpw` commands are unchanged. Started with `--user`, the script only runs the command. It is in `/usr/local/bin`, not `/app`, which belongs to `webhooker`.

README: the UID 1000 ownership block, the upaas pre-deploy commands and the restore ownership step are gone; the upaas volume bullet names only the path.

- Judgement call: `su-exec` over `setpriv`: Alpine's small tool for this, needing only musl; busybox's `setpriv` cannot change user, and util-linux's adds `libcap-ng`.
- Deviation: `su-exec` is pinned by version (`0.2-r3`), not by hash; `ca-certificates` beside it is unpinned.
- Judgement call: each start reads every entry's owner but changes only entries owned by someone else.
- `docker exec` and the health check now run as root, since the image sets no `USER`.
- No automated test covers the script: the suite runs inside `docker build`, which cannot start a container.
- A missing host directory under upaas is sneak/upaas#235.

Model: opus-5-5
Reviewed-on: #353
Co-authored-by: clawbot <35+clawbot@noreply.example.org>

deploy/docker-entrypoint.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+# deploy/docker-entrypoint.sh: the image's ENTRYPOINT. A bind-mounted
+# data directory keeps its owner from the host, often root, and the app
+# could not write to it. Started as root, this creates DATA_DIR if
+# needed, gives it and everything in it to webhooker, sets its mode, and
+# runs the command as webhooker, so the app never runs as root. Started
+# as another user, it only runs the command.
+set -eu
+
+main() {
+    if [ "$(id -u)" != 0 ]; then
+        exec "$@"
+    fi
+
+    dir="${DATA_DIR:-/var/lib/webhooker}"
+    mkdir -p "$dir"
+    find "$dir" ! -user webhooker -exec chown -h webhooker:webhooker {} +
+    chmod 750 "$dir"
+    exec su-exec webhooker "$@"
+}
+
+main "$@"