build: add devcontainer configurations

[keturn]

Here are devcontainer configurations to help provide dev environments.

[keturn]

These are working for me, but there are a few considerations:

• Contributes to Dockerfile proliferation. There are already too dang many Dockerfiles, some of which are falling out of sync with each other, and Dockerfiles lack a good mechanism to factor out the common bits. Reasons for adding new Dockerfiles for this were:
  • Removes the COPY of app/ sources, as sources will be mounted for development.
  • Adds non-root user. This was more to work around some third-party tooling issues, though depending on preference, it's something the production containers could do too.
• Quick Start documents using uv sync instead of uv pip install, so may be the Dockerfiles should be consistent with that? I would have gone ahead and made that change, but I know that doing the CUDA/CPU dependency switching with uv gets complicated, so I left that alone.
• While devcontainers can make use of Docker Compose, there really aren't multiple services to compose for this project, so I didn't add new compose files. Setting up mounts and environment variables and ports can all be done without that.