Only execute build-only step when not building+publishing as well

.drone.yml

@@ -5,20 +5,22 @@ type: docker
 name: default
 
 steps:
-  - name: build
+  - name: build only
     image: plugins/docker
     settings:
       dry_run: yes
-      purge: no
       repo: git.luj0ga.de/luca/matrix-prometheus
+    when:
+      event:
+        exclude:
+          - tag
 
-  - name: publish
+  - name: build and publish
     image: plugins/docker
     settings:
       auto_tag: yes
       password:
         from_secret: access_token
-      purge: no
       registry: git.luj0ga.de
       repo: git.luj0ga.de/luca/matrix-prometheus
       username: _