Container sets up its own data directory (closes #75) (#76)

Closes #75.

`bin/entrypoint.sh`, which already runs as root, now makes the data directory usable before the backend starts: it creates `DATA_DIR` if missing, gives it and `/data` to the `netwatch` user (`chown -R`), and sets mode 750 on both, the mode the backend gives a directory it creates. The backend still runs as `netwatch`. The README "Running under upaas" section loses its first-run step that created and chowned the host directory and names only the path to mount. The Dockerfile's build-time `mkdir` and `chown` of `/data` are gone, since the entrypoint now does this on every start.

What the diff does not show:

- The host directory mounted at `/data` ends up owned by uid 1000 with mode 750, and everything under `DATA_DIR` is chowned to uid 1000 on every start.
- If the directory cannot be created or chowned, the container stops with that tool's error before either process starts.

Recorded runs with `--mount type=bind`: an empty directory owned by root (mode 755, and again mode 700), and one holding a `reports` directory and report file owned by uid 1001 with mode 700. Each time the container turned healthy, `netwatch-server` ran as `netwatch`, and a posted report was written to `DATA_DIR`; a second start on the root-owned and the uid 1001 directories did the same.

Judgement call: `/data` itself is given to `netwatch` as well as `DATA_DIR`, so the backend can reach `DATA_DIR` inside a host directory with mode 700.

Model: opus-5-5
Reviewed-on: #76
Co-authored-by: clawbot <35+clawbot@noreply.example.org>

bin/entrypoint.sh

@@ -61,6 +61,29 @@ for proxy in $(printf '%s' "$TRUSTED_PROXIES" | tr ',' ' '); do
     echo "set_real_ip_from $cidr;"
 done > /etc/nginx/trusted-proxies.conf
 
+# netwatch-server keeps its report files in DATA_DIR, on the /data
+# volume, which may be a host directory owned by root or by another
+# uid. Both are given to the netwatch user here, with the mode the
+# server gives a directory it creates, so the host directory needs no
+# preparing.
+#
+# chown and chmod, run as root, change whatever a symbolic link on the
+# path points to, anywhere in the container, and the netwatch user can
+# put one in /data. So the start stops unless readlink -f, which
+# follows every link on a path, gives /data and DATA_DIR back as they
+# are. It also writes a path in full, so a DATA_DIR with '.', '..' or
+# an extra '/' in it is refused too.
+export DATA_DIR="${DATA_DIR:-/data/reports}"
+mkdir -p "$DATA_DIR" || exit 1
+if [ "$(readlink -f /data)" != /data ] ||
+    [ "$(readlink -f "$DATA_DIR")" != "$DATA_DIR" ]; then
+    echo "entrypoint: DATA_DIR must be a full path with no '.', '..'," \
+        "extra '/' or symbolic link on it or on /data, not '$DATA_DIR'" >&2
+    exit 1
+fi
+chown -R netwatch:netwatch /data "$DATA_DIR" || exit 1
+chmod 750 /data "$DATA_DIR" || exit 1
+
 # A stop signal is only noted here; the loop below acts on it.
 stop_requested=""
 trap 'stop_requested=yes' TERM INT