Fix .Net SDK Version (#14875)

Author: AlexanderSher

eng/containers/UbuntuNetCore2Keyring/Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/core/sdk:2.1-bionic AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:3.1-bionic AS build
 
 ENV \
     NO_AT_BRIDGE=1 \
@@ -18,4 +18,4 @@ RUN wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsof
     && dpkg -i packages-microsoft-prod.deb \
     && apt-get update \
     && apt-get install -y powershell \
-    && pwsh --version
+    && pwsh --version