dev: Add a justfile to gh-gei to simplify building and testing

Update the application to make the interaction with use a more cross-platform
build process, while still relying on the underlying dotnet build tools for the
implementation.

This is more discoverable; you can see what the project can do by running `just --choose`
which will offer a complete list of build commands.

- [-] Did you write/update appropriate tests
- [-] Release notes updated (if appropriate)
- [-] Appropriate logging output
- [-] Issue linked
- [X] Docs updated (or issue created)
- [-] New package licenses are added to `ThirdPartyNotices.txt` (if applicable)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: offbyone

.devcontainer/Dockerfile

@@ -0,0 +1,9 @@
+# Start from the .NET 8 devcontainer image
+FROM mcr.microsoft.com/devcontainers/dotnet:9.0
+
+# Set up apt and install Just as a task runner
+RUN apt-get update \
+    && apt-get install -y curl ca-certificates gnupg \
+    && curl --proto '=https' --tlsv1.2 -sSf https://just.systems/install.sh | bash -s -- --to /usr/local/bin \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*

.devcontainer/devcontainer.json

@@ -1,21 +1,23 @@
 {
 	"name": "C# (.NET)",
-	"image": "mcr.microsoft.com/devcontainers/dotnet:8.0",
+	"build": {
+		"dockerfile": "Dockerfile"
+	},
 
 	// Configure tool-specific properties.
 	"customizations": {
-		// Configure properties specific to VS Code.
-		"vscode": {	
-			// Add the IDs of extensions you want installed when the container is created.
+		"vscode": {
 			"extensions": [
 				"ms-dotnettools.csdevkit",
 				"github.copilot",
 				"ms-vscode.powershell"
 			]
 		}
 	},
+
 	"features": {
 		"ghcr.io/devcontainers/features/github-cli:1": {}
 	},
+
 	"remoteUser": "vscode"
-}
+}