Apply XDT transforms outside of Visual Studio.
The below is a demonstration outlining how the application may be called and what the inputs/outputs might look like.
XdtTransform --source SOURCE_WEB_CONFIG ^
--transform TRANSFORM_WEB_CONFIG ^
--destination DESTINATION_WEB_CONFIG
"SOURCE_WEB_CONFIG" (e.g. $(ProjectDir)\web.config)
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<connectionStrings>
<add name="default" connectionString="productionConnectionString" />
</connectionStrings>
</configuration>
"TRANSFORM_WEB_CONFIG" (e.g. $(ProjectDir)\web.$(Configuration).config)
<?xml version="1.0" encoding="utf-8"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
<connectionStrings>
<add name="default" connectionString="debugConnectionString"
xdt:Transform="Replace" xdt:Locator="Match(name)"/>
</connectionStrings>
</configuration>
"DESTINATION_WEB_CONFIG" (e.g. $(OutDir)\web.config)
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<connectionStrings>
<add name="default" connectionString="debugConnectionString" />
</connectionStrings>
</configuration>