Add a project-level Dev Container config

This should allow us to test the features, and potentially develop them
along the way.

Author: nickcharlton

.devcontainer/devcontainer.json

@@ -0,0 +1,9 @@
+{
+  "name": "Dev Containers",
+  "image": "ghcr.io/thoughtbot/devcontainers/images/debian:latest",
+  "features": {
+    "ghcr.io/devcontainers/features/docker-in-docker:4": {},
+    "ghcr.io/thoughtbot/devcontainers/features/asdf:1": {},
+    "ghcr.io/thoughtbot/devcontainers/features/asdf-plugin-nodejs:1": {},
+  }
+}