Add dry-run Docker build to Pull Request CI run

.drone.yml

@@ -26,6 +26,16 @@ steps:
       - pipenv run flake8
       - pipenv run reuse lint
       - pipenv run python -m unittest discover ki
+  - name: docker-dry-run
+    image: plugins/docker:20.18.4@sha256:a8d3d86853c721492213264815f1d00d3ed13f42f5c1855a02f47fa4d5f1e042
+    settings:
+      registry: git.wtf-eg.de
+      repo: git.wtf-eg.de/kompetenzinventar/backend
+      target: ki-backend
+      dry_run: true
+    when:
+      event:
+        - pull_request
 
 ---
 kind: pipeline