Add devcontainer infra to diagnostics repo (#2986)

.devcontainer/Dockerfile

@@ -0,0 +1,12 @@
+# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.192.0/containers/dotnet/.devcontainer/base.Dockerfile
+# For specifics about the dotnet base container see: https://github.com/microsoft/vscode-dev-containers/tree/main/containers/dotnet
+
+# [Choice] .NET version: 6.0, 5.0, 3.1, 2.1
+ARG VARIANT="6.0-focal"
+FROM mcr.microsoft.com/vscode/devcontainers/dotnet:0-${VARIANT}
+
+# Set up machine requirements to build the repo
+RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
+    && apt-get -y install --no-install-recommends cmake clang \
+    curl gdb gettext git libicu-dev lldb liblldb-dev libunwind8 \
+    llvm make python python-lldb tar wget zip 

.devcontainer/devcontainer.json

@@ -0,0 +1,37 @@
+// For format details, see https://aka.ms/devcontainer.json. For config options, see the README at:
+// https://github.com/microsoft/vscode-dev-containers/tree/v0.192.0/containers/dotnet
+{
+	"name": "C# (.NET)",
+	"build": {
+		"dockerfile": "Dockerfile",
+		"args": {
+			// Update 'VARIANT' to pick a .NET Core version: 2.1, 3.1, 5.0, 6.0
+			"VARIANT": "6.0-focal",
+		}
+	},
+
+	"settings": {
+		// Loading projects on demand is better for larger codebases
+		"omnisharp.enableMsBuildLoadProjectsOnDemand": true,
+		"omnisharp.enableRoslynAnalyzers": true,
+		"omnisharp.enableEditorConfigSupport": true,
+		"omnisharp.enableAsyncCompletion": true,
+	},
+
+	// Add the IDs of extensions you want installed when the container is created.
+	"extensions": [
+		"ms-dotnettools.csharp",
+		"ms-vscode.cpptools-extension-pack"
+	],
+
+	// Add the locally installed dotnet to the path to ensure that it is activated
+	// This allows developers to just use 'dotnet build' on the command-line, and the local dotnet version will be used.
+	// Add the global tools dir to the PATH so that globally installed tools will work
+	"remoteEnv": {
+		"PATH": "${containerWorkspaceFolder}/.dotnet:${containerWorkspaceFolder}/.dotnet-tools-global:${containerEnv:PATH}",
+		"DOTNET_MULTILEVEL_LOOKUP": "0"
+	},
+
+	// Comment out to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
+	"remoteUser": "vscode"
+}

.gitignore

@@ -7,6 +7,7 @@
 *.sln.docstates
 .vs/
 *.VC.db
+.venv/
 
 # Build results
 [Aa]rtifacts/
@@ -22,6 +23,7 @@ x64/
 .packages/
 .tools/
 .vscode/
+build/
 
 # Per-user project properties
 launchSettings.json