build: Add toolchain

toolchain/README.md

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+# Toolchain Docker image
+
+Based on the [Trimui toolchain Docker image](https://git.crowdedwood.com/trimui-toolchain/) by neonloop.
+
+## Installation
+
+With Docker installed and running, `make shell` builds the toolchain and drops into a shell inside the container. The container's `~/workspace` is bound to `./workspace` by default. The toolchain is located at `/opt/` inside the container.
+
+After building the first time, unless a dependency of the image has changed, `make shell` will skip building and drop into the shell. Running `make shell` from another window while already in a running shell will attach to the already running image.
+
+## Workflow
+
+- On your host machine, clone repositories into `./workspace` and make changes as usual.
+- In the container shell, find the repository in `~/workspace` and build as usual.
+
+See [setup-env.sh](./support/setup-env.sh) for some useful vars for compiling that are exported automatically.
+
+## Docker for Mac
+
+Docker for Mac has a memory limit that can make the toolchain build fail. Follow [these instructions](https://docs.docker.com/docker-for-mac/) to increase the memory limit. Also, docker on the M1 currently has a bug that stops reporting build progress after 5 minutes or so. The initial build takes about 15 minutes on my M1 so just be patient once you see `[output clipped, log limit 1MiB reached]`, it should get there.