Purge config doesn't work in drone-ecr

Hi, I couldn’t get the purge option to work with drone-ecr plugin.

The config option was a part of PR 142. Reading the source code shows that it is a drone-docker option, which should be inherited by drone-ecr as well.

Yet this is my relevant drone config:

  image: plugins/ecr                                                                                                                    
  access_key: XXX
  secret_key: XXX
  region: ap-southeast-1                                                                                                                
  repo: XXX
  registry: XXX
  purge: false                                                                                                                          
  tags: "${DRONE_COMMIT_SHA}"                                                                                                           
  storage_path: /drone/docker                                                                                                           
  use_cache: true                                                                                                                       

And I see this at the tail end of my log:

9612bb1c7201662aab1146393de78288c2e1c36d: digest: sha256:78e6f3b7988ebdc188f16557627b4903723ef1126fd7d3c6202758e1112867fa size: 5133
+ /usr/local/bin/docker rmi 9612bb1c7201662aab1146393de78288c2e1c36d
Untagged: 9612bb1c7201662aab1146393de78288c2e1c36d:latest
+ /usr/local/bin/docker system prune -f
Total reclaimed space: 0B 

Can anybody confirm?