Getting started - installation: provide instructions for container in Github codespaces

[radeksvarz]

Current instructions inform about Docker: https://book.getfoundry.sh/getting-started/installation#using-foundry-with-docker

However when such docker image is used in Github codespaces devcontainer configuration, the git is not available.

Example devcontainer (with git issue):

// For format details, see https://aka.ms/devcontainer.json. For config options, see the
// README at: https://github.com/devcontainers/templates/tree/main/src/alpine
{
	"name": "Foundry Development Environment",
	// Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
	"image": "ghcr.io/foundry-rs/foundry:latest",

	// Features to add to the dev container. More info: https://containers.dev/features.
	// "features": {},
	
	// Use 'forwardPorts' to make a list of ports inside the container available locally.
	// "forwardPorts": [],

	// Use 'postCreateCommand' to run commands after the container is created.
	"postCreateCommand": "forge install",

	// Configure tool-specific properties.
	"customizations": {
		"vscode": {
			"extensions": [
			  "JuanBlanco.solidity" // Solidity extension for VS Code
			  // Add other extensions as needed
			]
		  }		
	}

	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
	// "remoteUser": "root"
}