Stage 9: CI build on a self-hosted Gitea runner (0.7.0)

- .gitea/workflows/build.yml builds the packages and both ISOs with
  `make iso iso-server` on pushes to main and dev, v* tags and on demand
- ci/runner: setup of the CI host (WSL2 Ubuntu 24.04 with Docker Engine
  and gitea-runner as a systemd service, autostart task for Windows)
- scripts/builder.sh: DASOS_OUT keeps out/ outside the checkout, so CI
  reuses void-packages, the masterdir and the download cache
- README and TODO updated

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -0,0 +1,27 @@
+# Builds the Das OS packages and both ISO images on the CI host (see ci/runner).
+# Build output stays on the host in DASOS_OUT, set in the runner's config.
+name: build
+
+on:
+  push:
+    branches: [main, dev]
+    tags: ["v*"]
+  workflow_dispatch:
+
+concurrency:
+  group: build-${{ gitea.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: dasci
+    steps:
+      - uses: builtin:checkout
+
+      # One make run builds the packages once for both images.
+      # Go tests run inside the das-tools and das-theme package builds.
+      - name: Packages and ISO images
+        run: make iso iso-server
+
+      - name: Result
+        run: ls -lh "$DASOS_OUT/iso/"*-"$(cat VERSION)"-x86_64.iso