docker: add canonical docker images, devcontainer support, and offline export workflow

[jgoppert]

Closes #80

Summary

This PR adds the first complete Docker packaging stack for Rumoca and wires it into CI
and release workflows without bloating the main CI path.

It introduces a canonical image hierarchy:

• foundation: minimal shared base layer
• core: Rumoca runtime with Tier 1 backend dependencies
• ci: core plus OpenModelica and CI-only validation tooling
• dev: developer image with Rust, Node, wasm tooling, Jupyter, and OMC

It also adds:

• VS Code devcontainer wiring
• committed smoke tests for foundation, core, ci, dev, and offline dev round-trip
• offline export/load scripts, with dev as the default exported target
• a dedicated Docker publish workflow for nightly/manual/release publishing to GHCR
• release tarball upload for the canonical offline dev image

What changed

Images

• Added packaging/docker/Dockerfile
• Added packaged entrypoint and Docker docs under packaging/docker/
• Added image targets:
  • foundation
  • core
  • ci
  • dev

Runtime/tooling content

• core includes Tier 1 backend runtimes:
  • CasADi
  • Julia / SciML / ModelingToolkit
  • ONNX / ONNX Runtime
  • PyTorch
  • SymPy
• ci adds OpenModelica
• dev adds:
  • Rust nightly toolchain
  • rustfmt, clippy, rust-src
  • wasm32-unknown-unknown
  • wasm-bindgen-cli
  • wasm-pack
  • Node/npm
  • Jupyter / Notebook / ipykernel
  • contributor native build tools

Smoke coverage

• Added committed smoke scripts for:
  • foundation
  • core
  • ci
  • dev
  • offline dev export/load round-trip
• ci smoke includes a real offline OpenModelica compile/simulate check
• core smoke includes:
  • Python backend import/runtime checks
  • Julia ModelingToolkit/SciML DAE smoke

Devcontainer

• Added .devcontainer/devcontainer.json
• Points at the packaged dev image target
• Configures Python, Julia, shell, and baseline VS Code extensions

Offline workflow

• Added:
  • packaging/docker/export-image.sh
  • packaging/docker/load-image.sh
• Default export target is dev
• Supports canonical export/import of foundation, core, ci, and dev

CI / publishing

• Added Docker verification job in ci.yml for packaged ci smoke
• Added separate docker-publish.yml workflow for:
  • nightly/manual branch image publishing
  • tagged release image publishing
  • release dev tarball upload
• Kept Docker publishing out of the main CI/release critical path

Size notes

Measured image sizes during validation:

• foundation: ~156 MB
• core: ~4.30 GB
• ci: ~4.99 GB
• dev: ~6.59 GB

The stack is intentionally split so the heavier tooling does not inflate the baseline
runtime image.

Validation

Ran locally during this series:

• bash packaging/docker/smoke/foundation.sh
• bash packaging/docker/smoke/core.sh
• bash packaging/docker/smoke/ci.sh
• bash packaging/docker/smoke/dev.sh
• bash packaging/docker/smoke/offline-dev.sh

Also validated:

• docker-publish.yml and ci.yml workflow syntax
• offline export/import of the canonical dev image
• real OMC execution inside the ci image

Not in this PR

• multi-arch image support
• GPU-specific dev image
• GHCR/release refinements beyond the current nightly/manual/tagged flow

Those are deferred to later roadmap slices.