Build a reproducible Modal Image and avoid unnecessary rebuilds

Documented steps

1. Start from a base: `image = modal.Image.debian_slim(python_version="3.13")` (or `modal.Image.micromamba()`), then chain builder methods.
2. Install Python dependencies with `image.uv_pip_install("torch==2.8.0", "transformers==4.57.0")` — the uv-backed installer is the current preferred path; `pip_install(...)` remains available.
3. Add OS packages with `.apt_install("git", "ffmpeg")`, environment variables with `.env({"HF_HOME": "/cache"})`, and arbitrary shell steps with `.run_commands("...")`.
4. Run a Python function at build time (e.g. to pre-download weights) with `.run_function(download_model, secrets=[...], volumes={...})`.
5. Add local code and assets with `.add_local_file(local, remote_path=...)`, `.add_local_dir(...)`, `.add_local_python_source("mypkg")`. Pass copy=True to bake them into a build layer when later build steps must see them.
6. Order layers from most stable to most volatile: base -> apt -> pinned deps -> build steps -> local source. Each builder call is its own cached layer.
7. Declare imports shared by several functions with the `with image.imports():` context manager so they run in the container, not locally.
8. When a layer must be rebuilt, add `force_build=True` to that single call, deploy once, then remove it. `MODAL_FORCE_BUILD=1` rebuilds everything; `MODAL_IGNORE_CACHE=1` rebuilds without poisoning the cache.
9. Reference: https://modal.com/docs/guide/images