Add dockerfile for easier building on MacOS

tools/devenv.dockerfile

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+FROM rust:1.86-bullseye
+
+RUN apt-get update
+RUN apt-get install -y build-essential libc6-armhf-cross libc6-dev-armhf-cross gcc-arm-linux-gnueabihf
+RUN rustup target add armv7-unknown-linux-gnueabihf

tools/run-docker-devenv

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+#!/bin/sh
+
+# Tool to cross-compile outside of Linux.
+#
+# On MacOS, OrbStack is recommended, but other docker distributions will work
+# too.
+#
+# Usage:
+# ./tools/run-docker-devenv
+#
+# Inside the shell:
+# cargo build --bin rayhunter-daemon --target armv7-unknown-linux-gnueabihf --release
+#
+# Your output binary is in ./target/armv7-unknown-linux-gnueabihf/release/rayhunter-daemon
+
+docker build -t rayhunter-devenv -f tools/devenv.dockerfile .
+exec docker run --user $UID:$GID -v ./:/workdir -w /workdir -it rayhunter-devenv "$@"