Add Drone CI

.drone.yml

@@ -0,0 +1,19 @@
+---
+kind: pipeline
+name: deploy to swarm-test.autonomic.zone
+steps:
+  - name: deployment
+    image: decentral1se/stack-ssh-deploy:latest
+    settings:
+      host: swarm-test.autonomic.zone
+      stack: postfix-relay
+      purge: true
+      deploy_key:
+        from_secret: drone_ssh_swarm_test
+    environment:
+      DOMAIN: swarm-test.autonomic.zone
+      STACK_NAME: postfix-relay
+      LETS_ENCRYPT_ENV: production
+trigger:
+  branch:
+    - main

README.md

@@ -32,7 +32,7 @@ that in terms of reverse DNS?
 ## Using this in other stacks
 
 Add the other service to the (internal) `mail` network, and set whatever options
-the image supports for auto-configuring SMTP settings. 
+the image supports for auto-configuring SMTP settings.
 
 For reusable stacks, you probably want to put this configuration in a separate
 `compose.mailrelay.yml` file, e.g. [the version in