diff --git a/.gitea/workflows/build.yaml b/.gitea/workflows/build.yaml
index 294fcbf..5f67843 100644
--- a/.gitea/workflows/build.yaml
+++ b/.gitea/workflows/build.yaml
@@ -2,16 +2,13 @@ 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)
+# Runs on the dedicated host runner (label "yocto:host", Ryzen 9 box).
+# Everything persists in $YOCTO_HOME: extern/ (layer clones) + build/ (sstate)
+# survive between runs -> incremental builds after the first.
+# $YOCTO_HOME defaults to $HOME/yocto-ci; override it via runner.envs
+# (runner.envs.YOCTO_HOME=/data/yocto-ci) to put it on the big disk.
 #
-# Upgrade path: register a dedicated runner with label "yocto:host" and
-# change runs-on below to "yocto".
+# First build takes hours (poky fetch + full image) -> runner.timeout >= 8h.
 
 on:
   push:
@@ -28,7 +25,7 @@ permissions:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: yocto
     timeout-minutes: 600
     steps:
       - uses: builtin:checkout
@@ -52,7 +49,7 @@ jobs:
             echo "::error::Missing host tools:$missing"
             echo "Install on the runner host, e.g.:"
             echo '  sudo apt-get install -y --no-install-recommends build-essential \
-                  bison chrpath curl diffstat file gawk git libpython3-dev libssl-dev \
+                  bison flex chrpath curl diffstat file gawk git libpython3-dev libssl-dev \
                   make python3 python3-venv python3-pip python3-dev rsync swig unzip \
                   wget xz-utils zstd bzip2 nodejs npm'
             exit 1
@@ -61,7 +58,7 @@ jobs:
       - name: Set up kas (persistent venv)
         run: |
           set -e
-          export YOCTO_HOME="$HOME/yocto-ci"
+          export YOCTO_HOME="${YOCTO_HOME:-$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
@@ -70,7 +67,7 @@ jobs:
       - name: Sync to persistent project dir and build
         run: |
           set -euo pipefail
-          export YOCTO_HOME="$HOME/yocto-ci"
+          export YOCTO_HOME="${YOCTO_HOME:-$HOME/yocto-ci}"
           # Persistent project dir: keeps extern/ (layer clones) and build/
           # (sstate + tmp) between runs -> incremental builds after the first.
           rsync -a --delete \
@@ -83,7 +80,7 @@ jobs:
       - name: Collect images
         run: |
           set -euo pipefail
-          export YOCTO_HOME="$HOME/yocto-ci"
+          export YOCTO_HOME="${YOCTO_HOME:-$HOME/yocto-ci}"
           cd "$YOCTO_HOME/yocto_rpi"
           IMG_DIR="build/*/tmp/deploy/images/raspberrypi4-64"
           ls -lh $IMG_DIR