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)

README.md

@@ -60,6 +60,42 @@ still has an older version of sdcc, but you will need sdcc version 4.5 for the c
 sudo apt install make gcc sdcc xxd python-is-python3 libjson-c-dev
 ```
 
+<details>
+<summary>If using Docker (click to expand)</summary>
+
+A Dockerfile is provided for a reproducible build environment:
+
+```
+docker build -t rtlplayground-dev .
+```
+
+Build the firmware (replace MACHINE with your target, e.g. `DEFAULT_8C_1SFP`):
+
+```
+docker run --rm -v $(pwd):/workspace rtlplayground-dev make MACHINE=DEFAULT_8C_1SFP
+```
+
+The resulting `.bin` file appears in `output/` on your host.
+
+Build host tools only:
+
+```
+docker run --rm -v $(pwd):/workspace rtlplayground-dev make -C tools
+```
+
+Run the web-interface simulator locally:
+
+```
+docker run --rm -p 8080:8080 -v $(pwd):/workspace rtlplayground-dev \
+  tools/output/httpd_sim /workspace/html
+```
+
+Edit `machine.h` or `config.txt` on your host, then re-run `make` — the
+source directory is mounted into the container, so changes take effect
+immediately. To build for a different machine, pass `MACHINE=...`.
+
+</details>
+
 ## (1) Compiling for direct chip flashing AND upgrading an existing RTLPlayground running device
 
 Edit machine.h with an editor like vi or nano. Select the correct machine the firmware should build for.