Temporary fix for Linux CI build

Appveyor currently uses a Mono version (6.8) that is affected by a
regression which fails our CI test run. We need to fall back to an
older version. The last one known to not cause problems is 6.0.0.319.

Uninstalling Mono on Appveyor's Ubuntu images turns out to be rather
difficult, so I am giving up that approach for the moment. Let's use
Docker to get a clean build environment.

Unfortunately, this means having to install everything ourselves
(Mono, .NET Core SDK and runtime), which adds several 1,000 lines of
uninteresting garbage to the build log. But it appears to work.

We might want to create a clean Docker image that has all dependencies
preinstalled.

appveyor.yml

@@ -20,9 +20,10 @@ for:
 
   # build and run tests
   build_script:
-  - uname -a
-  - mono --version
-  - ./build.sh
+  - docker run --rm -i -v "$APPVEYOR_BUILD_FOLDER":/project ubuntu:18.04 /project/build.sh
+  # - uname -a
+  # - mono --version
+  # - ./build.sh
 
   # upload test results
   after_build:

build.sh

@@ -14,6 +14,35 @@
 # limitations under the License.
 # ****************************************************************************
 
+set -ex
+
+# Install Mono:
+apt-get update
+DEBIAN_FRONTEND=noninteractive apt-get -y install gnupg ca-certificates
+apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic/snapshots/6.0.0.319 main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+apt update
+DEBIAN_FRONTEND=noninteractive apt -y install mono-devel=6.0.0.*
+
+# Install .NET Core SDK:
+DEBIAN_FRONTEND=noninteractive apt-get -y install software-properties-common wget
+wget https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
+dpkg -i packages-microsoft-prod.deb
+add-apt-repository universe
+apt-get update
+DEBIAN_FRONTEND=noninteractive apt-get -y install apt-transport-https
+DEBIAN_FRONTEND=noninteractive apt-get -y install dotnet-sdk-3.1
+
+# Install .NET Core 1.1 runtime:
+apt-get update
+DEBIAN_FRONTEND=noninteractive apt-get -y install libunwind-dev
+cd /usr/share/dotnet
+wget https://download.visualstudio.microsoft.com/download/pr/d3bbce9c-a15e-4224-a271-064546459e53/694a1c880d7140c80215acb5dceab3db/dotnet-ubuntu.18.04-x64.1.1.13.tar.gz -O dotnet-ubuntu.18.04-x64.1.1.13.tar.gz
+tar xvzf dotnet-ubuntu.18.04-x64.1.1.13.tar.gz
+
+
+cd /project
+
 DOTNETPATH=$(which dotnet)
 if [ ! -f "$DOTNETPATH" ]; then
 	echo "Please install Microsoft/netcore from: https://www.microsoft.com/net/core"