fix: pin per-distro Docker builds to built-in node

Build #179 failed mid-pipeline with:
  failed to connect to the docker API at unix:///var/run/docker.sock:
  ... no such file or directory
because the unlabeled node() for the per-distro branches landed on a
Kubernetes pod (label 'jenkins-agent') that has no Docker socket.

Checked every agent in the pool: the registered 'local_docker' Docker
Cloud has zero templates (dead config, can never provide an agent);
fallgold has no Docker at all; tulameen has Docker but is arm64 (wrong
arch for these amd64 build images). Only the built-in node has a
working amd64 Docker daemon, so pin the per-distro stages there too —
consistent with the Aptly-publish stage, which was already pinned.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Vitexus

debian/Jenkinsfile

@@ -47,7 +47,7 @@ distributions.each { distro ->
         def artifacts = []
         def buildVer = ''
 
-        node {
+        node('built-in') {
             ansiColor('xterm') {
                 stage('Checkout ' + distroName) {
                     sh 'sudo chown -R jenkins:jenkins . 2>/dev/null || true'