Update plugins/docker Docker tag

Author: renovate[bot]

.drone.yml

@@ -51,7 +51,7 @@ steps:
     commands:
       - go build -o release/windows/amd64/buildx-gar.exe ./cmd/drone-buildx-gar
   - name: build gar plugin
-    image: plugins/docker:20.17.4-windows-1809-amd64
+    image: plugins/docker:20.18.6-windows-1809-amd64
     settings:
       dockerfile: docker/gar/Dockerfile.windows.amd64.1809
       repo: plugins/buildx-gar
@@ -149,7 +149,7 @@ steps:
      event:
        - tag
  - name: publish
-   image: plugins/docker:18
+   image: plugins/docker:20
    settings:
      auto_tag: true
      auto_tag_suffix: linux-amd64
@@ -209,7 +209,7 @@ steps:
        - tag
 
  - name: publish
-   image: plugins/docker:18
+   image: plugins/docker:20
    settings:
      auto_tag: true
      auto_tag_suffix: linux-arm64