Publish to IIS, setting Environment Variable

This answer was originally written for ASP.NET Core RC1. In RC2 ASP.NET Core moved from generic httpPlafrom handler to aspnetCore specific one. Note that step 3 depends on what version of ASP.NET Core you are using.

Turns out environment variables for ASP.NET Core projects can be set without having to set environment variables for user or having to create multiple commands entries.

1. Go to your application in IIS and choose Configuration Editor.
2. Select Configuration Editor
3. Choose system.webServer/aspNetCore (RC2 and RTM) or system.webServer/httpPlatform (RC1) in Section combobox
4. Choose Applicationhost.config ... in From combobox.
5. Right click on enviromentVariables element, select 'environmentVariables' element, then Edit Items.
6. Set your environment variables.
7. Close the window and click Apply.
8. Done

This way you do not have to create special users for your pool or create extra commands entries in project.json. Also, adding special commands for each environment breaks "build once, deploy many times" as you will have to call dnu publish separately for each environment, instead of publish once and deploying resulting artifact many times.

Updated for RC2 and RTM, thanks to Mark G and tredder.

Update web.config with an <environmentVariables> section under <aspNetCore>

<configuration>
  <system.webServer>
    <aspNetCore .....>
      <environmentVariables>
        <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Development" />
      </environmentVariables>
    </aspNetCore>
  </system.webServer>
</configuration>

Or to avoid losing this setting when overwriting web.config, make similar changes to applicationHost.config specifying the site location as @NickAb suggests.

<location path="staging.site.com">
  <system.webServer>
    <aspNetCore>
      <environmentVariables>
        <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Staging" />
      </environmentVariables>
    </aspNetCore>
  </system.webServer>
</location>
<location path="production.site.com">
  <system.webServer>
    <aspNetCore>
      <environmentVariables>
        <environmentVariable name="ASPNETCORE_ENVIRONMENT" value="Production" />
      </environmentVariables>
    </aspNetCore>
  </system.webServer>
</location>