Wait for docker daemon before build (fix container-start race)

Build #7 failed with "Cannot connect to the Docker daemon at
tcp://localhost:2375" right at the first sh step. Verified the dind
entrypoint script directly (docker-library/docker's
dockerd-entrypoint.sh) — with DOCKER_TLS_CERTDIR="" and a
dash-prefixed arg it correctly builds
`dockerd --host=tcp://0.0.0.0:2375 --insecure-registry=...`, so the
--insecure-registry flag added last commit isn't logically wrong.
This is a container-start race instead: Kubernetes doesn't guarantee
ordering between containers in the same pod, so the docker-cli
container's first sh can fire before dockerd in the sibling container
has finished its startup checks (iptables detection etc. run on every
start). Polls `docker info` for up to 60s before the actual build
instead of assuming instant availability.

src/com/homelab/stages/buildDocker.groovy

@@ -24,6 +24,7 @@ def run(Map config) {
     try {
         stage(stageName('Build & push image')) {
             container('docker-cli') {
+                waitForDockerDaemon()
                 dir("${config.repo_name}") {
                     if (!fileExists('Dockerfile')) {
                         renderDockerfile(config)
@@ -43,6 +44,25 @@ def run(Map config) {
     return tag
 }
 
+// Kubernetes doesn't guarantee container-start ordering within a pod, so
+// the sibling `docker` (dind) container can still be initializing
+// (iptables detection, cert-dir checks, etc. run on every start) when
+// this container's very first `sh` step fires — hit exactly this as
+// "Cannot connect to the Docker daemon at tcp://localhost:2375" on a
+// build that had worked fine moments earlier. Poll rather than assume
+// instant availability.
+def waitForDockerDaemon() {
+    sh '''
+        for i in $(seq 1 30); do
+            docker info >/dev/null 2>&1 && exit 0
+            echo "Waiting for docker daemon to be ready... ($i/30)"
+            sleep 2
+        done
+        echo "docker daemon never became ready" >&2
+        exit 1
+    '''
+}
+
 // Only reached when the repo has no Dockerfile of its own. Requires
 // config.dockerBuildVersion (e.g. 'go-1.22') — there's no sensible
 // default to fall back to if a repo has neither a Dockerfile nor this