Update dockerfile for net6.0 (#38645)

Author: christothes

eng/containers/UbuntuNetCoreKeyring/Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/powershell:7.2.1-ubuntu-focal AS build
+FROM mcr.microsoft.com/azure-powershell:10.3.0-ubuntu-20.04 AS build
 
 ENV \
     NO_AT_BRIDGE=1 \
@@ -8,7 +8,7 @@ ENV \
     # Do not generate certificate
     DOTNET_GENERATE_ASPNET_CERTIFICATE=false \
     # SDK version
-    DOTNET_SDK_VERSION_5_0=5.0.404 \
+    DOTNET_SDK_VERSION_6_0=6.0.413 \
     DOTNET_SDK_VERSION_3_1=3.1.416 \
     # Enable correct mode for dotnet watch (only mode supported in a container)
     DOTNET_USE_POLLING_FILE_WATCHER=true \
@@ -18,7 +18,7 @@ ENV \
     POWERSHELL_DISTRIBUTION_CHANNEL=PSDocker-DotnetSDK-Ubuntu-20.04 \
     # Setup Dotnet envs
     DOTNET_ROOT=/usr/share/dotnet  \
-    PATH=$PATH:usr/share/dotnet    
+    PATH=$PATH:usr/share/dotnet
 
 # Install apt-add-repository
 RUN apt-get update && apt-get install -y software-properties-common
@@ -39,8 +39,8 @@ RUN apt-add-repository ppa:git-core/ppa \
 
 # Below adapated from https://hub.docker.com/_/microsoft-dotnet-sdk
 # https://github.com/dotnet/dotnet-docker/blob/b20c03e0644b42167d66a85fe6077ec2428a47fa/src/sdk/5.0/focal/amd64/Dockerfile
-RUN curl -fSL --output dotnet.tar.gz https://dotnetcli.azureedge.net/dotnet/Sdk/$DOTNET_SDK_VERSION_5_0/dotnet-sdk-$DOTNET_SDK_VERSION_5_0-linux-x64.tar.gz \
-    && dotnet_sha512='6f9b83b2b661ce3b033a04d4c50ff3a435efa288de1a48f58be1150e64c5dd9d6bd2a4bf40f697dcd7d64ffaac24f14cc4a874e738544c5d0e8113c474fd2ee0' \
+RUN curl -fSL --output dotnet.tar.gz https://dotnetcli.azureedge.net/dotnet/Sdk/$DOTNET_SDK_VERSION_6_0/dotnet-sdk-$DOTNET_SDK_VERSION_6_0-linux-x64.tar.gz \
+    && dotnet_sha512='ee0a77d54e6d4917be7310ff0abb3bad5525bfb4beb1db0c215e65f64eb46511f5f12d6c7ff465a1d4ab38577e6a1950fde479ee94839c50e627020328a702de' \
     && echo "$dotnet_sha512  dotnet.tar.gz" | sha512sum -c - \
     && mkdir -p /usr/share/dotnet \
     && tar -ozxf dotnet.tar.gz -C /usr/share/dotnet \