Updating UbuntuNetCoreKeyring dockerfile to use net core 5 (Azure#17980)

* Updating UbuntuNetCoreKeyring dockerfile to use net core 5

* Update ubuntu netcore keyring dockerfile to use net 5

* adding python to installed packages

Co-authored-by: Ben Broderick Phillips <bebroder@microsoft.com>

Author: schaabs

eng/containers/UbuntuNetCoreKeyring/Dockerfile

@@ -1,23 +1,24 @@
-FROM mcr.microsoft.com/dotnet/core/sdk:3.1-bionic AS build
+FROM mcr.microsoft.com/dotnet/sdk:5.0-focal AS build
 
 ENV \
     NO_AT_BRIDGE=1 \
     DOCKER_CONTAINER_NAME="ubuntu_netcore_keyring" \
     PSModuleAnalysisCachePath=/var/cache/microsoft/powershell/PSModuleAnalysisCache/ModuleAnalysisCache \
-    POWERSHELL_DISTRIBUTION_CHANNEL=PSDocker-DotnetCoreSDK-Ubuntu-18.04
+    POWERSHELL_DISTRIBUTION_CHANNEL=PSDocker-DotnetCoreSDK-Ubuntu-20.04
 
 # Install GNOME keyring
 RUN apt-get update \
     && apt-get install -y \
         libsecret-1-dev \
         dbus-x11 \
-        gnome-keyring
+        gnome-keyring \
+        python
 
 # Install PowerShell && .net core runtime 2.1
-RUN wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb \
+RUN wget -q https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb \
     && dpkg -i packages-microsoft-prod.deb \
     && apt-get update \
     && apt-get install -y powershell \
     && pwsh --version \
     && apt-get install -y dotnet-runtime-2.1 \
-    && dotnet --list-runtimes
+    && dotnet --list-runtimes