Fix up devcontainer config (REUSE, strict JSON, image bump) and mention it in docs

- add SPDX licensing information for REUSE compliance
- drop the JSONC comment from devcontainer.json, keeping it strict JSON
  (as required by the check-json pre-commit hook)
- bump the base image to the current ghcr.io/devture/ansible release
- mention the dev container in docs/ansible.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -1,4 +1,3 @@
-// For format details, see https://aka.ms/devcontainer.json.
 {
 	"name": "matrix-docker-ansible-deploy",
 	"build": {