Web.csproj

  <PropertyGroup>
	<EnvConfig>$(EnvConfig)</EnvConfig>
	<EnvConfig Condition=" '$(EnvConfig)' == '' ">Local</EnvConfig>
  </PropertyGroup>
...
    <Content Include="Web.Production.config">
      <DependentUpon>Web.config</DependentUpon>
    </Content>
    <Content Include="Web.Test.config">
      <DependentUpon>Web.config</DependentUpon>
    </Content>
...
  <Target Name="AfterBuild" Condition="Exists('Web.$(Configuration).config')">
    <!--Generate transformed config in the output directory-->
    <TransformXml Source="Web.config" Destination="$(OutputPath)Web.config" Transform="Web.$(Configuration).config" />
    <TransformXml Condition="Exists('Web.$(EnvConfig).config')" Source="$(OutputPath)Web.config" Destination="$(OutputPath)Web.config" Transform="Web.$(EnvConfig).config" />
  </Target>
...

Then use msbuild to pass in the necessary params:
"C:\Program Files (x86)\MSBuild\14.0\Bin\msbuild.exe" ../MEDIC.Web.csproj /t:rebuild /p:Configuration=Release /p:EnvConfig="Production"

This will do a build and produce a Web.config file that has been transformed with the Web.Release.config and then the Web.Production.config similar to how Octopus does it