Update plugins/docker Docker tag to v21

Author: renovate[bot]

.drone.yml

@@ -76,7 +76,7 @@ steps:
   - ./release/linux/amd64/drone-docker --help
 
 - name: publish
-  image: plugins/docker:20
+  image: plugins/docker:21
   settings:
     auto_tag: true
     auto_tag_suffix: linux-amd64
@@ -162,7 +162,7 @@ steps:
     - tag
 
 - name: publish
-  image: plugins/docker:20
+  image: plugins/docker:21
   settings:
     auto_tag: true
     auto_tag_suffix: linux-amd64
@@ -247,7 +247,7 @@ steps:
     - tag
 
 - name: publish
-  image: plugins/docker:20
+  image: plugins/docker:21
   settings:
     auto_tag: true
     auto_tag_suffix: linux-amd64
@@ -333,7 +333,7 @@ steps:
     - tag
 
 - name: publish
-  image: plugins/docker:20
+  image: plugins/docker:21
   settings:
     auto_tag: true
     auto_tag_suffix: linux-amd64
@@ -418,7 +418,7 @@ steps:
     - tag
 
 - name: publish
-  image: plugins/docker:20
+  image: plugins/docker:21
   settings:
     auto_tag: true
     auto_tag_suffix: linux-amd64