more dockerfile polish

tools/dockerfile/distribtest/csharp_jessie_x64/Dockerfile

@@ -14,14 +14,13 @@
 
 FROM debian:jessie
 
+RUN apt-get update && apt-get install -y apt-transport-https && apt-get clean
+
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-RUN echo "deb http://download.mono-project.com/repo/debian jessie main" | tee /etc/apt/sources.list.d/mono-official.list
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy-apache24-compat main" | tee -a /etc/apt/sources.list.d/mono-xamarin.list
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy-libjpeg62-compat main" | tee -a /etc/apt/sources.list.d/mono-xamarin.list
+RUN echo "deb https://download.mono-project.com/repo/debian stable-jessie main" | tee /etc/apt/sources.list.d/mono-official-stable.list
 
 RUN apt-get update && apt-get install -y \
     mono-devel \
-    ca-certificates-mono \
     nuget \
     && apt-get clean
 

tools/dockerfile/distribtest/csharp_jessie_x86/Dockerfile

@@ -14,14 +14,13 @@
 
 FROM 32bit/debian:jessie
 
+RUN apt-get update && apt-get install -y apt-transport-https && apt-get clean
+
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-RUN echo "deb http://download.mono-project.com/repo/debian jessie main" | tee /etc/apt/sources.list.d/mono-official.list
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy-apache24-compat main" | tee -a /etc/apt/sources.list.d/mono-xamarin.list
-RUN echo "deb http://download.mono-project.com/repo/debian wheezy-libjpeg62-compat main" | tee -a /etc/apt/sources.list.d/mono-xamarin.list
+RUN echo "deb https://download.mono-project.com/repo/debian stable-jessie main" | tee /etc/apt/sources.list.d/mono-official-stable.list
 
 RUN apt-get update && apt-get install -y \
     mono-devel \
-    ca-certificates-mono \
     nuget \
     && apt-get clean
 

tools/dockerfile/distribtest/csharp_ubuntu1404_x64/Dockerfile

@@ -21,7 +21,6 @@ RUN echo "deb https://download.mono-project.com/repo/ubuntu stable-trusty main"
 
 RUN apt-get update && apt-get install -y \
     mono-devel \
-    ca-certificates-mono \
     nuget \
     && apt-get clean
 

tools/dockerfile/distribtest/csharp_ubuntu1604_x64/Dockerfile

@@ -21,7 +21,6 @@ RUN echo "deb https://download.mono-project.com/repo/ubuntu stable-xenial main"
 
 RUN apt-get update && apt-get install -y \
     mono-devel \
-    ca-certificates-mono \
     nuget \
     && apt-get clean