Add GitLabCI Linux pipeline

This pipeline doesn't come with Mono out-of-the-box like the GH-Actions
one, so then we need to install mono ourselves (and there are 3
different ways to do it).

.gitlab-ci.yml

@@ -0,0 +1,49 @@
+variables:
+  NUGET_540_URL: https://dist.nuget.org/win-x86-commandline/v5.4.0/nuget.exe
+  NUGET_451_URL: https://dist.nuget.org/win-x86-commandline/v4.5.1/nuget.exe
+
+before_script:
+  - apt update
+  # needed to download NuGet
+  - apt install -y curl
+
+stages:
+  - build
+
+stockmono_build:
+  image: ubuntu:18.04
+  stage: build
+  script:
+    # https://askubuntu.com/a/1013396
+    - DEBIAN_FRONTEND=noninteractive apt install -y mono-complete mono-xbuild fsharp
+    - mono --version
+
+    - curl -o nuget.exe $NUGET_451_URL
+    - mono nuget.exe restore src/TgSharp.sln
+    - xbuild src/TgSharp.Core/TgSharp.Core.csproj
+
+stocknewmono_build:
+  image: ubuntu:20.04
+  stage: build
+  script:
+    # https://askubuntu.com/a/1013396
+    - DEBIAN_FRONTEND=noninteractive apt install -y mono-complete mono-xbuild fsharp
+    - mono --version
+
+    - curl -o nuget.exe $NUGET_540_URL
+    - mono nuget.exe restore src/TgSharp.sln
+    - xbuild src/TgSharp.Core/TgSharp.Core.csproj
+
+newmono_build:
+  image: ubuntu:18.04
+  stage: build
+  artifacts:
+    paths:
+      - bin/*.zip
+    expire_in: 50days
+  script:
+    - ./scripts/install_mono_from_microsoft_deb_packages.sh
+
+    - curl -o nuget.exe $NUGET_540_URL
+    - mono nuget.exe restore src/TgSharp.sln
+    - msbuild src/TgSharp.Core/TgSharp.Core.csproj

scripts/install_mono_from_microsoft_deb_packages.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euxo pipefail
+
+# required by apt-key
+apt install -y gnupg2
+# required by apt-update when pulling from mono-project.com
+apt install -y ca-certificates
+
+# taken from http://www.mono-project.com/download/stable/#download-lin
+apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+# bionic(=18.04) below works even for 18.10, 19.04 and 19.10
+echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+apt update
+apt install -y mono-devel
+mono --version