feat: add Docker build environment

Ubuntu 24.04 container with ARM cross-compilation toolchain for
building the ROM without host toolchain setup. Includes Makefile
targets for docker-build, docker-shell, and docker-clean.

Co-Authored-By: Claude

.dockerignore

@@ -0,0 +1,5 @@
+.git
+.gitignore
+Dockerfile
+docker-compose.yml
+build/**

DOCKER.md

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+# Docker
+
+Build pokeemerald without installing the toolchain on your host machine.
+
+## Prerequisites
+
+- [Docker](https://docs.docker.com/get-docker/) with Compose v2
+
+## Usage
+
+```bash
+# Build the ROM
+make docker-build
+
+# Open a shell inside the container
+make docker-shell
+
+# Clean build artifacts
+make docker-clean
+```
+
+The output ROM is written to `pokeemerald_modern.gba` in the project root.
+
+## How it works
+
+The container is an Ubuntu 24.04 image with the ARM cross-compilation toolchain (`gcc-arm-none-eabi`, `binutils-arm-none-eabi`) and build dependencies pre-installed. Your project directory is volume-mounted into the container at `/workspace`, so source edits on the host are immediately reflected.
+
+The build uses `MODERN=1`, which compiles with the system `arm-none-eabi-gcc` rather than the legacy `agbcc` compiler. A persistent Docker volume caches `ccache` data across builds to speed up recompilation.
+
+On first run, Docker will build the image - subsequent runs reuse the cached image.

Dockerfile

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+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV PATH="/usr/lib/ccache:${PATH}"
+
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    binutils-arm-none-eabi \
+    gcc-arm-none-eabi \
+    git \
+    make \
+    python3 \
+    ccache \
+    libpng-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV CCACHE_DIR=/ccache
+
+WORKDIR /workspace
+
+CMD ["bash"]

Makefile

@@ -389,3 +389,15 @@ $(ROM): $(ELF)
 # Symbol file (`make syms`)
 $(SYM): $(ELF)
 	$(OBJDUMP) -t $< | sort -u | grep -E "^0[2389]" | $(PERL) -p -e 's/^(\w{8}) (\w).{6} \S+\t(\w{8}) (\S+)$$/\1 \2 \3 \4/g' > $@
+
+# Docker
+.PHONY: docker-build docker-shell docker-clean
+
+docker-build:
+	HOST_UID=$$(id -u) HOST_GID=$$(id -g) docker compose run --rm pokeemerald
+
+docker-shell:
+	HOST_UID=$$(id -u) HOST_GID=$$(id -g) docker compose run --rm pokeemerald bash
+
+docker-clean:
+	HOST_UID=$$(id -u) HOST_GID=$$(id -g) docker compose run --rm pokeemerald make clean

docker-compose.yml

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+services:
+  pokeemerald:
+    build: .
+    working_dir: /workspace
+    environment:
+      CCACHE_DIR: /ccache
+    volumes:
+      - ./:/workspace
+      - pokeemerald_ccache:/ccache
+    command: bash -lc "make clean-tools && make MODERN=1 -j$$(nproc)"
+
+volumes:
+  pokeemerald_ccache: