Container sets its data directory's owner and mode itself (closes #340)

The image now starts as root through deploy/docker-entrypoint.sh,
which creates DATA_DIR if it is missing, gives the directory and
anything in it owned by another user to webhooker, sets the directory
to 0750, and runs the command as webhooker with su-exec. An empty
root-owned bind mount, or data left by another uid, now works with no
step on the host. Started with --user, the script only runs the
command.

The README drops every instruction to create or chown the host
directory; the upaas volume bullet names only the path.

Model: opus-5-5

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 "$@"
+}
+
+main "$@"