Add support for both .Net Core 2.1 and .Net Core 3.1

Author: AlexanderSher

eng/containers/UbuntuNetCore2Keyring/Dockerfile renamed to eng/containers/UbuntuNetCoreKeyring/Dockerfile

@@ -2,7 +2,7 @@ FROM mcr.microsoft.com/dotnet/core/sdk:3.1-bionic AS build
 
 ENV \
     NO_AT_BRIDGE=1 \
-    DOCKER_CONTAINER_NAME="ubuntu_netcore2_keyring" \
+    DOCKER_CONTAINER_NAME="ubuntu_netcore_keyring" \
     PSModuleAnalysisCachePath=/var/cache/microsoft/powershell/PSModuleAnalysisCache/ModuleAnalysisCache \
     POWERSHELL_DISTRIBUTION_CHANNEL=PSDocker-DotnetCoreSDK-Ubuntu-18.04
 
@@ -13,9 +13,11 @@ RUN apt-get update \
         dbus-x11 \
         gnome-keyring
 
-# Install PowerShell
+# Install PowerShell && .Net SDK 2.1
 RUN wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb \
     && dpkg -i packages-microsoft-prod.deb \
     && apt-get update \
     && apt-get install -y powershell \
-    && pwsh --version
+    && pwsh --version \
+    && apt-get install -y dotnet-sdk-2.1 \
+    && dotnet --list-sdks

eng/containers/ci.yml

@@ -12,9 +12,9 @@ pool:
   vmImage: 'ubuntu-18.04'
 
 variables:
-  dockerfile: ./eng/containers/UbuntuNetCore2Keyring/Dockerfile
+  dockerfile: ./eng/containers/UbuntuNetCoreKeyring/Dockerfile
   containerRegistry: 'azsdkengsys'
-  imageRepository: 'dotnet/ubuntu_netcore2_keyring'
+  imageRepository: 'dotnet/ubuntu_netcore_keyring'
   imageTag: $(build.buildid)
 
 steps: