Prefer to run mono via Docker

This lets us choose which version of Mono to be use without having to
install it.

This is important because the version currently preinstalled on App-
Veyor has a regression that breaks our build, and installing another
Mono version on AppVeyor is both slow and error-prone.

README.md

@@ -48,7 +48,7 @@ Most of these requirements should be covered by Visual Studio 2017.
 
 Compilation requires an up-to-date .NET Core SDK.
 
-Running the unit tests additionally requires Mono and the .NET Core 1.1 runtime to be installed. We recommend Mono 5.10+, though older versions (4.6.1+) might still work as well.
+Running the unit tests additionally requires the .NET Core 1.1 runtime to be installed, as well as either Docker or Mono. For the latter, we recommend Mono 5.10+, though older versions (4.6.1+) might still work as well.
 
 :information_source: **Mono runtime support:** Castle Core runs with minor limitations and defects on Mono 4.0.2+ (however 4.6.1+ is highly recommended, or 5.10+ if your code uses new C# 7.x language features such as `in` parameters).
 

appveyor.yml

@@ -21,7 +21,6 @@ for:
   # build and run tests
   build_script:
   - uname -a
-  - mono --version
   - ./build.sh
 
   # upload test results

build.sh

@@ -13,19 +13,27 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 # ****************************************************************************
+shopt -s expand_aliases
 
 DOTNETPATH=$(which dotnet)
 if [ ! -f "$DOTNETPATH" ]; then
 	echo "Please install Microsoft/netcore from: https://www.microsoft.com/net/core"
 	exit 1
 fi
-MONOPATH=$(which mono)
 
-if [ ! -f "$MONOPATH" ]; then
-	echo "Please install Xamarin/mono from: http://www.mono-project.com/docs/getting-started/install/"
-	exit 1
+DOCKERPATH=$(which docker)
+if [ -f "$DOCKERPATH" ]; then
+	alias mono="$PWD/buildscripts/docker-run-mono.sh"
+else
+	MONOPATH=$(which mono)
+	if [ ! -f "$MONOPATH" ]; then
+		echo "Please install either Docker, or Xamarin/Mono from http://www.mono-project.com/docs/getting-started/install/"
+		exit 1
+	fi
 fi
 
+mono --version
+
 # Linux/Darwin
 OSNAME=$(uname -s)
 echo "OSNAME: $OSNAME"

buildscripts/docker-run-mono.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+set -e
+MONO_TAG=${MONO_TAG:-6.0.0.334}
+docker run --rm -v "$PWD":'/project' -w='/project' mono:$MONO_TAG mono "$@"