Ditch 5.x packages, we have no builds for Buster

5.20.1.34/Dockerfile

@@ -1,8 +0,0 @@
-FROM mono:5.20.1.34-slim
-
-# MAINTAINER Jo Shields <jo.shields@xamarin.com>
-# MAINTAINER Alexander Köplinger <alkpli@microsoft.com>
-
-RUN apt-get update \
-  && apt-get install -y binutils curl mono-devel ca-certificates-mono fsharp mono-vbnc nuget referenceassemblies-pcl \
-  && rm -rf /var/lib/apt/lists/* /tmp/*

5.20.1.34/slim/Dockerfile

@@ -1,22 +0,0 @@
-FROM debian:stretch-slim
-
-# MAINTAINER Jo Shields <jo.shields@xamarin.com>
-# MAINTAINER Alexander Köplinger <alkpli@microsoft.com>
-
-ENV MONO_VERSION 5.20.1.34
-
-RUN apt-get update \
-  && apt-get install -y --no-install-recommends gnupg dirmngr \
-  && rm -rf /var/lib/apt/lists/* \
-  && export GNUPGHOME="$(mktemp -d)" \
-  && gpg --batch --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF \
-  && gpg --batch --export --armor 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF > /etc/apt/trusted.gpg.d/mono.gpg.asc \
-  && gpgconf --kill all \
-  && rm -rf "$GNUPGHOME" \
-  && apt-key list | grep Xamarin \
-  && apt-get purge -y --auto-remove gnupg dirmngr
-
-RUN echo "deb http://download.mono-project.com/repo/debian stable-stretch/snapshots/$MONO_VERSION main" > /etc/apt/sources.list.d/mono-official-stable.list \
-  && apt-get update \
-  && apt-get install -y mono-runtime \
-  && rm -rf /var/lib/apt/lists/* /tmp/*