Match instance runner label; probe environment on first run

Host-based kas build on a dedicated 'yocto:host' runner with a
persistent project dir ($HOME/yocto-ci) for incremental sstate reuse.
Uploads deploy images from raspberrypi4-64 as artifacts.

.gitea/workflows/build.yaml

@@ -0,0 +1,104 @@
+name: Build RPi image
+
+# Yocto (kas) build of rpilinux-image for raspberrypi4-64.
+#
+# Runner requirements (see chat/notes):
+#   - matches the instance runner's "ubuntu-latest" label (bare label = job
+#     runs on the runner's machine, i.e. a host job). Yocto needs ~250GB
+#     persistent disk, so the runner must actually be a machine (or a
+#     container with a big persistent volume), not a throwaway sandbox.
+#   - ~250GB free in $HOME, plus the poky host dependencies + node
+#   - runner.timeout >= 8h for the first build (runner default is 3h)
+#
+# Upgrade path: register a dedicated runner with label "yocto:host" and
+# change runs-on below to "yocto".
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+# Never run two builds at once on the same (single) runner.
+concurrency:
+  group: yocto-rpi-build
+  cancel-in-progress: false
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 600
+    steps:
+      - uses: builtin:checkout
+
+      - name: Probe environment and host dependencies
+        run: |
+          echo "=== runner environment ==="
+          uname -a
+          [ -r /etc/os-release ] && . /etc/os-release && echo "$PRETTY_NAME"
+          echo "HOME=$HOME  nproc=$(nproc)"
+          df -h "$HOME" | tail -1
+          free -h 2>/dev/null | sed -n 2p || true
+          command -v docker >/dev/null && docker info --format 'docker: {{.ServerVersion}}' 2>/dev/null || echo "no docker daemon"
+          echo "=== dependency check ==="
+          set -e
+          missing=""
+          for t in git python3 curl wget rsync gawk xz diffstat chrpath node npm; do
+            command -v "$t" >/dev/null 2>&1 || missing="$missing $t"
+          done
+          if [ -n "$missing" ]; then
+            echo "::error::Missing host tools:$missing"
+            echo "Install on the runner host, e.g.:"
+            echo '  sudo apt-get install -y --no-install-recommends build-essential \
+                  bison chrpath curl diffstat file gawk git libpython3-dev libssl-dev \
+                  make python3 python3-venv python3-pip python3-dev rsync swig unzip \
+                  wget xz-utils zstd bzip2 nodejs npm'
+            exit 1
+          fi
+
+      - name: Set up kas (persistent venv)
+        run: |
+          set -e
+          export YOCTO_HOME="$HOME/yocto-ci"
+          mkdir -p "$YOCTO_HOME"
+          [ -d "$YOCTO_HOME/venv" ] || python3 -m venv "$YOCTO_HOME/venv"
+          "$YOCTO_HOME/venv/bin/pip" -q install -U kas
+          "$YOCTO_HOME/venv/bin/kas" --version
+
+      - name: Sync to persistent project dir and build
+        run: |
+          set -euo pipefail
+          export YOCTO_HOME="$HOME/yocto-ci"
+          # Persistent project dir: keeps extern/ (layer clones) and build/
+          # (sstate + tmp) between runs -> incremental builds after the first.
+          rsync -a --delete \
+            --exclude .git --exclude extern --exclude build \
+            ./ "$YOCTO_HOME/yocto_rpi/"
+          cd "$YOCTO_HOME/yocto_rpi"
+          git config --global --add safe.directory "$YOCTO_HOME/yocto_rpi" || true
+          "$YOCTO_HOME/venv/bin/kas" build
+
+      - name: Collect images
+        run: |
+          set -euo pipefail
+          export YOCTO_HOME="$HOME/yocto-ci"
+          cd "$YOCTO_HOME/yocto_rpi"
+          IMG_DIR="build/*/tmp/deploy/images/raspberrypi4-64"
+          ls -lh $IMG_DIR
+          mkdir -p "$GITEA_WORKSPACE/dist"
+          cp $IMG_DIR/* "$GITEA_WORKSPACE/dist/"
+          {
+            echo "## RPi4 image built"
+            echo
+            echo "- commit: ${GITEA_SHA}"
+            echo "- images also on the runner host at: \`$YOCTO_HOME/yocto_rpi/$IMG_DIR\`"
+          } >> "$GITEA_STEP_SUMMARY"
+
+      - name: Upload images
+        uses: actions/upload-artifact@v4
+        with:
+          name: rpilinux-image-rpi4
+          path: dist/
+          retention-days: 14