Add instructions for using Codespaces (#60996)

* Add instructions for using Codespaces

* Automate resetting the HEAD to the same commit ID used to build the pre-built container

* - Add notes about testing changes.

- Add 4-core machines

* Revert 4-core change since prebuild doesn't support multiple sku_names.

.devcontainer/devcontainer.json

@@ -23,6 +23,9 @@
 	// Use 'onCreateCommand' to run pre-build commands inside the codespace
 	"onCreateCommand": "${containerWorkspaceFolder}/.devcontainer/scripts/onCreateCommand.sh",
 
+	// Use 'postCreateCommand' to run commands after the container is created.
+	"postCreateCommand": "${containerWorkspaceFolder}/.devcontainer/scripts/postCreateCommand.sh",
+
 	// 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.
 	"remoteEnv": {

.devcontainer/scripts/onCreateCommand.sh

@@ -4,3 +4,6 @@ set -e
 
 # prebuild the repo, so it is ready for development
 ./build.sh libs+clr -rc Release
+
+# save the commit hash of the currently built assemblies, so developers know which version was built
+git rev-parse HEAD > ./artifacts/prebuild.sha

.devcontainer/scripts/postCreateCommand.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+set -e
+
+# reset the repo to the commit hash that was used to build the prebuilt Codespace
+git reset --hard $(cat ./artifacts/prebuild.sha)

docs/workflow/Codespaces.md

@@ -0,0 +1,34 @@
+# Using Codespaces
+Codespaces allows you to develop in a Docker container running in the cloud. You can use an in-browser version of VS Code or the full VS Code application with the [GitHub Codespaces VS Code Extension](https://marketplace.visualstudio.com/items?itemName=GitHub.codespaces). This means you don't need to install dotnet/runtime's prerequisites on your current machine in order to develop in dotnet/runtime.
+
+## Create a Codespace
+
+dotnet/runtime runs a nightly GitHub Action to build the latest code in the repo. This allows you to immediately start developing and testing after creating a codespace without having to build the whole repo. When the machine is created, it will have built the repo using the code as of 6 AM UTC that morning.
+
+**NOTE**: In order to use a prebuilt codespace, when you create your machine be sure to select an **`8 core`** machine.
+
+See https://docs.github.com/codespaces/developing-in-codespaces/creating-a-codespace#creating-a-codespace for instructions on how to create a new codespace.
+
+## Updating dotnet/runtime's Codespaces Configuration
+
+The Codespaces configuration is spread across the following places:
+
+1. The [.devcontainer](../../.devcontainer) folder contains:
+    - `devcontainer.json` file configures the codespace and mostly has VS Code settings
+    - The Dockerfile used to create the image
+    - The `scripts` folder contains any scripts that are executed during the creation of the codespace. This has the build command that builds the entire repo for prebuilds.
+2. The GitHub Action can be configured at [create-codespaces-prebuild](../../.github/workflows/create-codespaces-prebuild.yml)
+    - This contains when the Action is run, what regions we build prebuilds for, and what size machines
+
+To test out changes to the `.devcontainer` files, you can follow the process in [Applying changes to your configuration](https://docs.github.com/codespaces/customizing-your-codespace/configuring-codespaces-for-your-project#applying-changes-to-your-configuration) docs. This allows you to rebuild the Codespace privately before creating a PR.
+
+To test out your `.yml` changes, here is the process:
+
+**Note**: *Executing these steps will overwrite the current prebuilt container for the entire repo. Afterwards, anyone creating a new codespace will get a prebuilt machine with your test changes until the Action in `main` is executed again.*
+
+1. Edit and commit the files to a branch.
+2. Push that to a branch on dotnet/runtime. Be careful that you aren't pushing to `main` or some other important branch. Prefix your branch name with your GitHub account name, so others know it is a dev branch. ex. `username/FixCodespaces`.
+3. In the "Actions" tab at the top of dotnet/runtime:
+    - Select "Create Codespaces Prebuild" action on the left
+    - On the right click "Run workflow" and pick your branch
+    - After it runs, try to create a codespace

docs/workflow/README.md

@@ -46,6 +46,8 @@ To build just one part you use the root build script (build.cmd/sh), and you add
 
 For instructions on how to edit code and debug your changes, see [Editing and Debugging](editing-and-debugging.md).
 
+For instructions on using GitHub Codespaces, see [Codespaces](Codespaces.md).
+
 ## Configurations
 
 You may need to build the tree in a combination of configurations. This section explains why.