feat: add local Docker/Podman build option

Dockerfile

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+FROM docker.io/zmkfirmware/zmk-build-arm:stable
+
+RUN apt-get update \
+    && apt-get install -y wget \
+    && wget https://github.com/mikefarah/yq/releases/latest/download/yq_linux_amd64 -O /usr/bin/yq \
+    && chmod +x /usr/bin/yq
+
+ARG USER_ID=1000
+
+RUN adduser --disabled-password --gecos '' --uid ${USER_ID} zmk
+
+USER zmk
+
+WORKDIR /app
+
+COPY config/west.yml config/west.yml
+
+RUN mkdir -p build \
+    && west init -l config \
+    && west update \
+    && west zephyr-export
+
+COPY bin/build.sh ./
+
+CMD ["./build.sh"]

Makefile

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+DOCKER := $(shell { command -v podman || command -v docker; })
+
+.PHONY: all clean distclean
+
+all:
+	$(DOCKER) build --tag zmk-build-user-config --build-arg USER_ID=$(shell id -u) .
+	$(DOCKER) volume create zmk_build
+	$(DOCKER) run --rm -it --name zmk-build-user-config -u zmk \
+		-v zmk_build:/app/build \
+		-v $(PWD)/build.yaml:/app/build.yaml:ro \
+		-v $(PWD)/config:/app/config:ro \
+		-v $(PWD)/firmware:/app/firmware \
+		-e OUTPUT_ZMK_CONFIG=$(OUTPUT_ZMK_CONFIG) \
+		zmk-build-user-config
+
+clean:
+	rm -rf firmware/[^.]*
+	-$(DOCKER) volume rm zmk_build
+
+distclean: clean
+	-$(DOCKER) image rm zmk-build-user-config
+	-$(DOCKER) system prune

README.md

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+# Unified ZMK Config
+
+## Modifying the keymap
+
+[The ZMK documentation](https://zmk.dev/docs) covers both basic and advanced functionality and has a table of OS compatibility for keycodes.
+
+## Building the Firmware with GitHub Actions
+
+### Setup
+
+1. Fork this repo.
+2. Enable GitHub Actions on your fork.
+
+### Build firmware
+
+1. Push a commit to trigger the build.
+2. Download the artifact.
+
+## Building the Firmware in a local container
+
+### Setup
+
+#### Software
+
+* Either Podman or Docker is required, Podman is preferred if both are present.
+* `make` is also required
+
+#### Windows specific
+
+* If compiling on Windows use WSL2 and Docker [Docker Setup Guide](https://docs.docker.com/desktop/windows/wsl/).
+* Install `make` using
+    ```bash
+    sudo apt-get install make
+    ```.
+* The repository can be cloned directly into the WSL2 instance or accessed through the C: mount point WSL provides by default (`/mnt/c/path-to-repo`).
+
+### Build firmware
+
+1. Execute `make`.
+2. Check the `firmware` directory for the latest firmware build.
+
+#### Output ZMK Configs
+Sometimes it can be useful to output the full ZMK configs for a build. This can be achieved by executing the following:
+```bash
+OUTPUT_ZMK_CONFIG=1 make
+```
+
+The configs with then be outputted to the `firmware` directory along with the firmware files.
+
+#### Build Cache
+A docker volume is used to cache the build artifacts, allowing subsequent builds to compile faster.
+
+### Cleanup
+
+#### Partial Cleanup
+The compiled firmware files and docker volume build cache can be deleted with `make clean`. This might be necessary if you want a pristine build or to cleanup the `firmware` directory.
+
+### Full Cleanup
+
+The built docker container can be deleted with `make distclean`. This might be necessary if you want to rebuild the docker container or update the ZMK version used to compile your firmware.
+
+Of course, you can also use regular Docker commands to manage the image/build cache.
+
+## Flashing firmware
+
+Follow the ZMK instructions on [Flashing UF2 Files](https://zmk.dev/docs/user-setup#flashing-uf2-files) to flash the firmware.

bin/build.sh

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+#!/usr/bin/env bash
+
+config_dir="${PWD}/config"
+timestamp=$(date -u +"%Y%m%d%H%M%S")
+
+while IFS=$',' read -r board shield; do
+    extra_cmake_args=${shield:+-DSHIELD="$shield"}
+    artifact_name=${shield:+${shield// /-}-}${board}-zmk
+    filename="${PWD}/firmware/${timestamp}-${artifact_name}"
+    build_dir="${PWD}/build/${artifact_name}"
+
+    echo ""
+    echo "-----------------"
+    echo "BUILDING FIRMWARE"
+    echo "-----------------"
+    echo "Zephyr: ${ZEPHYR_VERSION}"
+    echo "Board: ${board}"
+    echo "Shield: ${shield}"
+    echo ""
+
+    if [[ ! -d "$build_dir" ]]; then
+        west build -s zmk/app -b "$board" -d ${build_dir} -- \
+            -DZMK_CONFIG="$config_dir" "${extra_cmake_args}"
+    else
+        west build -d ${build_dir} -- -DZMK_CONFIG="$config_dir"
+    fi
+
+    if [[ ! -z $OUTPUT_ZMK_CONFIG ]]; then
+        grep -v -e "^#" -e "^$" ${build_dir}/zephyr/.config \
+            | sort > "${filename}.config"
+    fi
+
+    extensions="uf2 hex bin"
+    for extension in $extensions; do
+        artifact=${build_dir}/zephyr/zmk.$extension
+        if [[ -f $artifact ]]; then
+            cp $artifact "${filename}.$extension"
+            break
+        fi
+    done
+done < <(yq '
+    [{"board": .board[], "shield": .shield[] // ""}] + .include
+    | filter(.board)
+    | unique_by(.board + .shield).[]
+    | [.board, .shield // ""]
+    | @csv
+' build.yaml)

firmware/.gitignore

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+*
+!.gitignore