docker: fix env var for colossus service

docker-compose.yml

@@ -44,7 +44,7 @@ services:
       dockerfile: apps.Dockerfile
     ports:
       - '127.0.0.1:3001:3001'
-    command: colossus --dev --ws-provider {WS_PROVIDER_ENDPOINT_URI} --ipfs-host ipfs
+    command: colossus --dev --ws-provider ${WS_PROVIDER_ENDPOINT_URI} --ipfs-host ipfs
     environment:
       - DEBUG=*