add ubuntu sources.list

env-example

@@ -69,6 +69,8 @@ COMPOSE_CONVERT_WINDOWS_PATHS=1
 
 # If you need to change the sources (i.e. to China), set CHANGE_SOURCE to true
 CHANGE_SOURCE=false
+# Set CHANGE_SOURCE and UBUNTU_SOURCE option if you want to change the Ubuntu system sources.list file.
+UBUNTU_SOURCE=aliyun
 
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