Add .devcontainer (#1032)

Adds support for [devcontainers](https://containers.dev/), this should
make onboarding easier and should allow (especially simpler)
contributions entirely online via Github Codespaces 🚀

.devcontainer/devcontainer.json

@@ -0,0 +1,14 @@
+// For format details, see https://aka.ms/devcontainer.json. For config options, see the
+// README at: https://github.com/devcontainers/templates/tree/main/src/docker-existing-docker-compose
+{
+	"name": "Actual development",
+	"dockerComposeFile": [
+		"../docker-compose.yml",
+		"docker-compose.yml"
+	],
+	// Alternatively:
+	// "image": "mcr.microsoft.com/devcontainers/typescript-node:0-16",
+	"service": "actual-development",
+	"workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}",
+	"postCreateCommand": "yarn install"
+}