'Unknown URL scheme unix' Exception when running with mono on Linux

**Output of `dotnet --info`:**

```
$ dotnet --info
.NET Command Line Tools (2.1.201)

Product Information:
Version: 2.1.201
Commit SHA-1 hash: 7932dc6179

Runtime Environment:
OS Name: ubuntu
OS Version: 16.04
OS Platform: Linux
RID: ubuntu.16.04-x64
Base Path: /home/phil/.dotnet/sdk/2.1.201/

Microsoft .NET Core Shared Framework Host

Version : 2.0.7
Build : 2d61d0b043915bc948ebf98836fefe9ba942be11
```

**What version of Docker.DotNet?:**

```
3.125.2
```

**Steps to reproduce the issue:**
1. Attempt to use the library to connect to the docker daemon on linux, building with mono (in a project targeting `net471`): `var dockerClient = new DockerClientConfiguration("unix:///var/run/docker.sock").CreateClient();`
2. Exception will be thrown.

**What actually happened?:**
An Exception with the message "Unknown URL scheme unix" was thrown.

**What did you expect to happen?:**
The connection to the docker daemon was made successfully, just as if building and running a dotnet core `netcoreapp2.0` project.