Fix env.FAILURE NPE and buildx read-only-mount failure

Build #5 got through checkout, loadConfig, and the pre_build hook,
then hit two real bugs at the actual docker build step:

1. `docker build` failed with "mkdir /root/.docker/buildx: read-only
   file system" — dind-pod.yaml mounts the Harbor push-auth secret
   read-only at /root/.docker, but modern docker defaults to
   BuildKit/buildx, which wants to write its own state there. Forces
   the classic builder via DOCKER_BUILDKIT=0 instead.

2. The subsequent error-handling itself then threw a
   NullPointerException — every stage file (including untouched
   legacy ones from the real devops-lib) reads env.FAILURE when
   setting currentBuild.result, but nothing in this repo ever defined
   it, so it was null. Defined once in homelabPipeline.groovy rather
   than touching 40+ individual occurrences across every stage file.

resources/org/homelab/dind-pod.yaml

@@ -28,6 +28,16 @@ spec:
       env:
         - name: DOCKER_HOST
           value: tcp://localhost:2375
+        # docker-config below mounts the Harbor push-auth secret
+        # read-only at /root/.docker (needed so `docker push` finds
+        # config.json without an explicit `docker login` step) — but
+        # modern `docker build` defaults to BuildKit/buildx, which wants
+        # to create its own state dir at /root/.docker/buildx and fails
+        # with "read-only file system" since the whole mount is
+        # read-only. Forcing the classic builder avoids needing to write
+        # there at all.
+        - name: DOCKER_BUILDKIT
+          value: "0"
         # For syncArgoApp.groovy — read directly from the ESO-managed
         # Secret, not a Jenkins-native credential (nothing in this
         # pipeline uses Jenkins' own credential store; staying consistent