Containerization baseline

Add `Dockerfile` using micromamba; install pipeline env (`envs/base.yml`/`r.yml`) and Snakemake.

- Copy repo into `/app`; default `CMD` starts the FastAPI server.
- Document docker `build`/`run` (`-p 8000:8000`).

Acceptance criteria:
- Pipeline can be driven via the containerized API/UI.