Add scheduled weekly build

.forgejo/workflows/build.yaml

@@ -1,11 +1,18 @@
 name: Build Docker image
 
-on: [push]
+on:
+  push:
+  schedule:
+    - cron: "0 6 * * 7"
 
 jobs:
   build_image:
     name: Build and publish image
     runs-on: docker
+    if: env.SCHEDULED == 'true' || !contains(github.event.commits[0].message, '[skip ci]')
+
+    env:
+      SCHEDULED: ${{ github.event_name == 'schedule' }}
 
     steps:
       - name: Set up Docker Buildx

README.adoc

@@ -17,6 +17,10 @@ was too old.
 ** npm and nodejs
 ** git
 
+== Updates
+
+This image is built weekly on sunday at 06:00 EET/EEST on the `archlinux:latest` image.
+
 == Installation
 
 * link:https://git.korhonen.cc/FunctionalHacker/-/packages/container/hugo-asciidoc-docker-ci/latest[Download Docker image]