Add shellchecking

.drone.yml

@@ -2,6 +2,12 @@
 kind: pipeline
 name: deploy to hub.docker.com
 steps:
+  - name: run shellcheck
+    image: debian:buster
+    commands:
+      - apt update
+      - apt install -y shellcheck
+      - shellcheck plugin.sh
   - name: docker push
     image: plugins/docker
     settings: