Added docker build environment

docker/README.md

@@ -0,0 +1,24 @@
+# QMK Build Container
+
+This container contains everything you need to build QMK, including a pinned version of QMK itself. **Be careful, this container may not use the latest version of QMK.**
+
+## Usage
+
+This container takes two volumes:
+ - `/build_output`: QMK binaries are copied here.
+ - `/qmk_firmware/keyboards`: QMK keyboard dir. Link this to your code.
+
+```bash
+docker run -it \
+	--user $(id -u):$(id -g) \
+	-v "$(pwd)/output:/build_output" \
+	-v "$(pwd)/kb:/qmk_firmware/keyboards:ro" \
+	-e QMK_TARGET="betalupi_ergodox:default" \
+	git.betalupi.com/mark/qmk
+```
+
+## Building
+
+1. Before building, run `git submodule update --init --recursive`.
+2. Checkout whatever version of `qmk_firmware` you need
+3. Run `docker build . -t git.betalupi.com/mark/qmk`