Pin Docker images in CI

.drone.yml

@@ -43,7 +43,7 @@ depends_on:
 
 steps:
   - name: docker-publish
-    image: plugins/docker
+    image: plugins/docker:20.17.3
     settings:
       registry: git.wtf-eg.de
       repo: git.wtf-eg.de/kompetenzinventar/backend
@@ -70,7 +70,7 @@ depends_on:
 
 steps:
   - name: deploy-dev
-    image: appleboy/drone-ssh
+    image: appleboy/drone-ssh:1.7.5
     settings:
       host:
         - dev01.wtf-eg.net
@@ -105,7 +105,7 @@ steps:
       - pipenv run reuse lint
       - pipenv run python -m unittest discover ki
   - name: docker-publish
-    image: plugins/docker
+    image: plugins/docker:20.17.3
     settings:
       registry: git.wtf-eg.de
       repo: git.wtf-eg.de/kompetenzinventar/backend