Update plugins/docker Docker tag to v20

Author: renovate[bot]

.drone.yml

@@ -106,7 +106,7 @@ steps:
      event:
        - tag
  - name: publish
-   image: plugins/docker:18
+   image: plugins/docker:20
    settings:
      auto_tag: true
      auto_tag_suffix: linux-amd64
@@ -166,7 +166,7 @@ steps:
        - tag
 
  - name: publish
-   image: plugins/docker:18
+   image: plugins/docker:20
    settings:
      auto_tag: true
      auto_tag_suffix: linux-arm64