Add a Dockerfile to the project to allow building on Linux

Author: rprouse

BUILDING.md

@@ -82,8 +82,10 @@ One convenient way to do this is to pass the same arguments to
 
 For example, to build and test everything: `.\build-mono-docker.ps1 -t test`
 
-This will run a temporary container using a custom
-[rprouse/nunit-docker](https://hub.docker.com/r/rprouse/nunit-docker/) image
+This will build a docker image and run a temporary container
 based on the [Mono image](https://hub.docker.com/r/library/mono/) and adding in
 .NET Core. The script mounts the repo inside the container and executes the
 [build.sh](build.sh) Cake bootstrapper with the arguments you specify.
+
+The first build will be slow as it builds the new image, but subsequent runs will
+be much quicker.

Dockerfile

@@ -0,0 +1,10 @@
+FROM mono:6.6.0
+
+# Install .NET Core
+RUN apt-get update && \
+    apt-get -y install wget && \
+    wget -q https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb && \
+    dpkg -i packages-microsoft-prod.deb && \
+    apt-get -y install apt-transport-https && \
+    apt-get update && \
+    apt-get -y install dotnet-sdk-2.2

build-mono-docker.ps1

@@ -1,7 +1,8 @@
-trap [Management.Automation.CommandNotFoundException] {
+if ($null -eq (Get-Command "docker.exe" -ErrorAction SilentlyContinue)) {
   Write-Error 'Docker cannot be found. Make sure it is installed and added to the path.'
   Start-Process -FilePath 'https://docs.docker.com/docker-for-windows/install/'
   continue;
 }
 
-docker run --rm -it -v ${PSScriptRoot}:/nunit-console -w=/nunit-console rprouse/nunit-docker:latest bash build.sh $args
+docker build -t nunit-console-build .
+docker run --rm -it -v ${PSScriptRoot}:/nunit-console -w=/nunit-console nunit-console-build bash build.sh $args