Ship a toolchain-python Docker image for cross-compilation

[ppenna]

Summary

Create and ship a derived Docker image that layers a specific Python version on top of ghcr.io/nanvix/toolchain-gcc, to be used as the cross-compilation environment for CPython on Nanvix.

Background

The Nanvix toolchain is migrating from nanvix/toolchain:latest-minimal (Docker Hub) to ghcr.io/nanvix/toolchain-gcc:sha-34a3641 (GHCR). The new GCC image is intentionally minimal — it contains only binutils + gcc + newlib for i686-nanvix.

However, CPython's cross-compilation requires a host Python 3 interpreter (passed via --with-build-python). The old image provided this at /opt/nanvix/bin/python3; the new GCC-only image does not.

Proposal

This repository should own and publish a derived Docker image, e.g.:

FROM ghcr.io/nanvix/toolchain-gcc:sha-34a3641

RUN apt-get update && apt-get install -y --no-install-recommends \
    python3 python3-dev \
    && rm -rf /var/lib/apt/lists/*

Published as: ghcr.io/nanvix/toolchain-python:<tag>

Requirements

• Base image: ghcr.io/nanvix/toolchain-gcc (pinned to a specific tag/SHA)
• Adds: Python 3 (matching or close to the CPython version being cross-compiled)
• Python should be available at a predictable path (e.g., /usr/bin/python3)
• Published to GHCR from this repo's CI

Files to Update After Image is Available

• Makefile.nanvix — update NANVIX_DOCKER_IMAGE default and BUILD_PYTHON path
• .nanvix/config.py — update DOCKER_IMAGE constant
• .nanvix/env.json (if present) — update NANVIX_DOCKER_IMAGE field
• NANVIX.md — update docker pull instructions

Notes

• The BUILD_PYTHON path in Makefile.nanvix currently points to /opt/nanvix/bin/python3. After migration, it should point to /usr/bin/python3 (or wherever the derived image installs it).
• This keeps the GCC toolchain image lean for all other repos while giving CPython exactly what it needs.