[bdbarnett]

You can now build firmware without installing a toolchain, in a GitHub codespace or a local dev container. Each port gets its own config, so building one port doesn't pull in the others' packages.

• .devcontainer/{unix,webassembly,rp2,windows,esp32}/devcontainer.json, one shared Dockerfile with a PORT build arg. The image runs install.sh --deps $PORT, which installs from the same package list install.sh prints for your own machine. build_mp.py still fetches each toolchain on its first build, into deps/. Each config keeps deps/ in a named volume, so rebuilding a container doesn't fetch the toolchain again.
• esp32 fetches MicroPython, the modules and ESP-IDF with its compilers in onCreateCommand, so a Codespaces prebuild of that config does the slow part ahead of time. Its compilers go into the volume through IDF_TOOLS_PATH.
• webassembly: scripts/serve_workbench.sh downloads the newest workbench release, swaps in your build's micropython.mjs and micropython.wasm, and serves it on port 8000. The config forwards that port and opens it in your browser.
• The Dev containers workflow builds each config from a clean image and runs one real build in it: unix and webassembly with every module (each then runs a script that imports displaydev), rp2 RPI_PICO2, windows, and esp32 ESP32_GENERIC_S3. The webassembly job also checks that workbench serves the build it just made. It runs when a config changes, and weekly.
• docs/newcomers.md gains "In a codespace", the README gets the Codespaces badge, and the roadmap gains "Install from a file".