Fix .NET Docker image used in Codespaces (#36843)

* Fix .NET Docker image used in Codespaces

* Update Dockerfile

Author: captainsafia

.devcontainer/Dockerfile

@@ -2,9 +2,9 @@
 
 # [Choice] .NET version: 5.0, 3.1, 2.1
 ARG VARIANT="5.0"
-FROM mcr.microsoft.com/vscode/devcontainers/dotnet:0-${VARIANT}
+FROM mcr.microsoft.com/vscode/devcontainers/dotnet
 
 # [Option] Install Node.js
 ARG INSTALL_NODE="true"
 ARG NODE_VERSION="lts/*"
-RUN if [ "${INSTALL_NODE}" = "true" ]; then su vscode -c "umask 0002 && . /usr/local/share/nvm/nvm.sh && nvm install ${NODE_VERSION} 2>&1"; fi
+RUN if [ "${INSTALL_NODE}" = "true" ]; then su vscode -c "umask 0002 && . /usr/local/share/nvm/nvm.sh && nvm install ${NODE_VERSION} 2>&1"; fi

.devcontainer/devcontainer.json

@@ -5,8 +5,12 @@
 	"build": {
 		"dockerfile": "Dockerfile",
 		"args": {
-			// Update 'VARIANT' to pick a .NET Core version: 2.1, 3.1, 5.0, 6.0, 7.0
-			"VARIANT": "7.0",
+			// The VARIANT here must align with a dotnet container image that
+			// is publicly available on https://mcr.microsoft.com/v2/vscode/devcontainers/dotnet/tags/list.
+			// We'll default to `latest` as the default. Generally, the .NET version that is baked
+			// into the image by default doesn't matter since we end up installing our own
+			// local version and using that by default in the container environment.
+			"VARIANT": "latest",
 			// Options
 			"INSTALL_NODE": "true",
 			"NODE_VERSION": "lts/*"