The new image restores the ability of SSH to ask about unknown host keys
and to prompt for the passphrase of an SSH key, which Ansible 2.21 broke
by having forked workers run session-isolated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

- 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>