Match instance runner label; probe environment on first run

.gitea/workflows/build.yaml

@@ -3,10 +3,15 @@ name: Build RPi image
 # Yocto (kas) build of rpilinux-image for raspberrypi4-64.
 #
 # Runner requirements (see chat/notes):
-#   - label "yocto:host"  (host job: Yocto needs ~250GB persistent disk,
-#     container jobs are not suitable)
+#   - 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:
@@ -23,13 +28,21 @@ permissions:
 
 jobs:
   build:
-    runs-on: yocto
+    runs-on: ubuntu-latest
     timeout-minutes: 600
     steps:
       - uses: builtin:checkout
 
-      - name: Probe host dependencies
+      - 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