feat: add Dockerfile for development environment

- Debian 13 (trixie) based with sdcc 4.5.0 from apt
- Includes gcc, make, xxd, python3, libjson-c-dev, golang-go
- Add .dockerignore to exclude build artifacts
- Add Docker usage section to README (collapsible)

Dockerfile

@@ -0,0 +1,19 @@
+FROM debian:13-slim
+
+RUN apt-get update && apt-get install -y \
+    make \
+    gcc \
+    sdcc \
+    xxd \
+    python3 \
+    libjson-c-dev \
+    golang-go \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+# git safe.directory for mounted repos (Makefile uses git describe)
+RUN git config --global --add safe.directory /workspace
+
+WORKDIR /workspace
+
+CMD ["bash"]