ci: Use dedicated slave instead of Docker

Using a dedicated slave node instead of a Docker container has a few
advantages:

* Persistent configuration is possible, without making weird assumptions
  (e.g. Jenkins UID/GID) in the Dockerfile
* Can limit concurrent deployments by controlling executor count on the
  node

ci/dch-gw.jenkinsfile

@@ -2,9 +2,7 @@
 
 pipeline {
     agent {
-        dockerfile {
-            dir 'ci'
-        }
+        label 'ansible'
     }
 
     triggers {