Setup devcontainer

Adding a devcontainer and docs on how to configure it would help ensure a consistent developer experience and provide a stable platform for onboarding new developers.

This would involve:

- Splitting the Dockerfile into a multi-stage build, with the stages useful for developing (checking out the code, configuring env vars) in a stage named developer: this also allows caching that layer of the build and speed up future builds.
- Configure a devcontainer.json to make use of the developer stage of the Dockerfile