try something

.drone.yml

@@ -4,6 +4,8 @@ type: docker
 name: default
 
 environment:
+  CREDS_JSON:
+    from_secret: CREDS_JSON
   HE_USERNAME:
     from_secret: HE_USERNAME
   HE_PASSWORD:
@@ -19,6 +21,7 @@ steps:
 - name: Validate configuration and check changes
   image: stackexchange/dnscontrol
   commands:
+    - echo $CREDS_JSON > creds.json
     - dnscontrol preview
 
 - name: Push new configuration to providers