Select an Ubuntu 18.04 base environment
Switch to the postInstall script and use the following:
#!/usr/bin/env bash
set -ex
buildDeps="apt-transport-https dirmngr ca-certificates"
apt update && apt install -y $buildDeps
apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb https://download.mono-project.com/repo/ubuntu vs-bionic main" | tee /etc/apt/sources.list.d/mono-official-vs.list
apt update && apt-get install -y monodevelop
apt-get purge -y $builodDeps && rm -rf /var/lib/apt/lists/*
You can check that this worked by running mono --version in your run script, which should give output like:
Mono JIT compiler version 5.18.1.28 (tarball Tue Jul 16 22:21:52 UTC 2019)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
Interpreter: yes
LLVM: yes(600)
Suspend: preemptive
GC: sgen (concurrent by default)