change dockerfile from mono:latest to ubuntu:20.04 and install mono manually

Author: FreeAndNil

Dockerfile

@@ -16,7 +16,21 @@
 
 # MAINTAINER Jan Friedrich
 
-FROM mono:latest
+FROM ubuntu:20.04
+ENV DEBIAN_FRONTEND=noninteractive \
+    TZ=Etc/UTC
+
+# Install Mono SDK (compiler, msbuild, runtime, etc.)
+RUN apt-get update && \
+    apt-get install -y 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-focal main" | tee /etc/apt/sources.list.d/mono-official-stable.list && \
+    apt-get update && \
+    apt-get install -y mono-complete && \
+    rm -rf /var/lib/apt/lists/*
+
+# Check Mono version
+RUN mono --version
 
 RUN apt-get update \
   && apt-get upgrade -y \