dry_run and cache_from its own image

.drone.jsonnet

@@ -15,9 +15,12 @@ local build(dir, package) = {
     dockerfile: '%(dir)s/%(package)s/Dockerfile' % { dir: dir, package: package },
     registry: 'http://cr.wks',
     insecure: 'true',
+    purge: 'false',
     experimental: 'true',
     tags: ['latest'],
     repo: 'cr.wks/%(package)s' % { package: package },
+    cache_from: 'cr.wks/%(package)s:latest' % { package: package },
+    dry_run: 'true'
   },
 };
 [
@@ -47,4 +50,4 @@ local build(dir, package) = {
       for app in apps
     ],
   },
-]
+]