Nuget command fails in docker file but not when run manually

I am trying to configure a container to run my build. (It is a windows core container.)

When I run a nuget command in the docker file, it fails. But when I connect powershell to the container, it runs fine.

This is the command in question:

nuget sources Add -Name "Common Source" -Source http://CompanyPackages/nuget/Common

I run it from the docker file like this:

RUN nuget sources Add -Name "Common Source" -Source http://CompanyPackages/nuget/Common

And get the following error:

sources: invalid arguments.

However, when I take the container and start it using:

docker run -it agent:v1

Then run the same command inside the container:

nuget sources Add -Name "Common Source" -Source http://CompanyPackages/nuget/Common

The result is:

Package Source with Name: Common added successfully.

What would cause it to fail in the dockerfile and not in the container?

Note:

In case it is useful, here is my full docker file:

 FROM sixeyed/msbuild:netfx-4.5.2-webdeploy AS build-agent
 SHELL ["powershell"]

 RUN nuget sources Add -Name "Common Source" -Source http://CompanyPackages/nuget/Common