Add runner devcontainer (actions#2187)

Add runner devcontainer.json definition. Sets up dotnet, docker-in-docker, omnisharp and some vscode extensions

.devcontainer/devcontainer.json

@@ -0,0 +1,24 @@
+// For format details, see https://aka.ms/devcontainer.json. For config options, see the README at:
+{
+	"name": "Actions Runner Devcontainer",
+	"image": "mcr.microsoft.com/devcontainers/base:focal",
+	"features": {
+		"ghcr.io/devcontainers/features/docker-in-docker:1": {},
+		"ghcr.io/devcontainers/features/dotnet": {
+			"version": "6.0.300"
+		}
+	},
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"ms-azuretools.vscode-docker",
+				"ms-dotnettools.csharp",
+				"eamodio.gitlens"
+			]
+		}
+	},
+	// dotnet restore to install dependencies so OmniSharp works out of the box
+	// src/Test restores all other projects it references, src/Runner.PluginHost is not one of them
+	"postCreateCommand": "dotnet restore src/Test && dotnet restore src/Runner.PluginHost",
+	"remoteUser": "vscode"
+}