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.

vars/homelabPipeline.groovy

@@ -27,6 +27,19 @@ def call(Map config) {
     config.helm_repo_url     = config.helm_repo_url      ?: 'http://gitea.192.168.1.7.nip.io/mukul/devops-helm-charts.git'
     config.image_tag_yq_path = config.image_tag_yq_path  ?: '.deployment.image.tag'
 
+    // Every stage file (both the ones adapted for this homelab and the
+    // untouched legacy ones carried over from the real devops-lib) reads
+    // `env.FAILURE` when setting currentBuild.result on error — the real
+    // system must define this as a global Jenkins environment variable
+    // somewhere upstream of buildPipeline.groovy, since it's referenced
+    // everywhere but never set anywhere in this repo. Left undefined,
+    // `env.FAILURE` is null, and `currentBuild.result = null` throws
+    // `NoSuchMethodError`/`NullPointerException` deep inside Jenkins'
+    // own result-normalization code — masking whatever the real error
+    // was. Defining it once here, before any stage runs, is the minimal
+    // fix rather than touching every individual stage file.
+    env.FAILURE = 'FAILURE'
+
     podTemplate(yaml: libraryResource('org/homelab/dind-pod.yaml')) {
         node(POD_LABEL) {
             def checkOutObj = new com.homelab.stages.checkOut()