Fix build so as not to attempt to build and push documentation for a non-master branch.

.drone.yml

@@ -78,6 +78,7 @@ steps:
       from_secret: docker_password
     repo: witten/borgmatic-docs
     dockerfile: docs/Dockerfile
-  when:
-    branch:
-      - master
+
+trigger:
+  branch:
+    - master

NEWS

@@ -1,5 +1,6 @@
 1.5.14.dev0
  * #398: Clarify canonical home of borgmatic in documentation.
+ * Fix build so as not to attempt to build and push documentation for a non-master branch.
 
 1.5.13
  * #373: Document that passphrase is used for Borg keyfile encryption, not just repokey encryption.