fix: [outline] align datastore ownership with the application user

Outline runs as an unprivileged user in the image (``nodejs`` since
1.10.0, ``root`` up to 1.6.1) while the datastore is provisioned by
``root``.  Without realignment the application cannot write its
``uploads``, ``public`` and ``avatars`` buckets, so every attachment
upload fails with "Permission denied writing to ... Check the host
machine file system permissions".  This was seen on elabore.coop
after the 1.6.1 to 1.10.0 upgrade.

The ``init`` hook now reads the image's ``Config.User`` and chowns the
datastore to that user, skipping root-based images.  It is idempotent
and version-agnostic: the realignment also covers buckets added by
future Outline versions.

outline/hooks/init

@@ -71,4 +71,42 @@ $SERVICE_NAME:
     #DEBUG: \"http\"
 "
 
+## The datastore is bind-mounted into the container.  Outline runs as
+## an unprivileged user (image Config.User: root up to 1.6.1, "nodejs"
+## since 1.10.0) and must write its uploads, public and avatars
+## buckets.  Provisioned by root, the datastore is not writable by
+## that user and every upload fails with "Permission denied writing
+## to ... Check the host machine file system permissions".  Align the
+## datastore ownership with the image user; skip images running as
+## root.  See README.org, "Datastore ownership alignment".
+app_user=
+if [ -n "$DOCKER_BASE_IMAGE" ]; then
+    app_user=$(docker image inspect "$DOCKER_BASE_IMAGE" \
+        --format '{{.Config.User}}') || exit 1
+fi
+
+case "$app_user" in
+    ""|0|0:0|root)
+        ## image runs as root: nothing to align
+        ;;
+    *:*)
+        uid="${app_user%%:*}"
+        gid="${app_user#*:}"
+        ;;
+    *)
+        uid_gid=($(docker_get_uid_gid "$SERVICE_NAME" "$app_user" "$app_user")) || exit 1
+        uid="${uid_gid[0]}"
+        gid="${uid_gid[1]}"
+        ;;
+esac
+
+if [ -n "${uid:-}" ]; then
+    mkdir -p "$SERVICE_DATASTORE"
+    chown -R "$uid:$gid" "$SERVICE_DATASTORE" || {
+        err "Failed to align datastore ownership on '$uid:$gid'."
+        exit 1
+    }
+    info "Datastore ownership aligned on '$uid:$gid'."
+fi
+