ci: add shellcheck step

.drone.yml

@@ -7,6 +7,11 @@ platform:
   arch: amd64
 
 steps:
+  - name: lint
+    image: koalaman/shellcheck-alpine:stable
+    commands:
+      - shellcheck ./wait-for
+
   - name: markdownlint
     image: thegeeklab/markdownlint-cli
     commands:
@@ -121,6 +126,6 @@ depends_on:
 
 ---
 kind: signature
-hmac: 3d3a49cd900fe822c6568d4f2f2e6e9d628ddf2aca147079658b117c8de92c45
+hmac: b39df187d1afbd5bcdd61c24ab2ec2754f545189fe1297e6b8f020d242c882b6
 
 ...