Add docker-in-docker feature to devcontainer

Enables building and serving docs as described in docs/src/develop.md

Author: ethho

.devcontainer/devcontainer.json

@@ -31,6 +31,7 @@
 	"onCreateCommand": "python3 -m pip install -e .",
 	"features": {
 		"ghcr.io/devcontainers/features/git:1": {},
+		"ghcr.io/devcontainers/features/docker-in-docker:2": {},
 	},
 	// Configure tool-specific properties.
 	"customizations": {