Move dev container Dockerfile into new repo

[lucyb]

The devcontainers Dockerfile, the process to build the Docker image and the associated documentation should live in a repo separate to the research-template. The repo is here: https://github.com/opensafely-core/research-template-docker

This repo must be added to the team-rex team, as we'll be responsible for it.

The repo should contain:

• The dockerfile
• A README
• An ADRs folder
• A GitHub workflow for linting the dockerfile and for building+uploading a new image when the dockerfile changes using a "latest" tag for now

Testing of the devcontainer config and docker image will be done as part of https://github.com/opensafely-core/codespaces-initiative/issues/18.