Update build instructions and Dockerfile to download submodules

Dockerfile

@@ -26,4 +26,4 @@ VOLUME /qmk_firmware
 WORKDIR /qmk_firmware
 COPY . .
 
-CMD make $KEYBOARD:$KEYMAP
+CMD make clean ; make git-submodule ; make $KEYBOARD:$KEYMAP

util/docker_build.sh

@@ -47,4 +47,4 @@ dir=$(pwd -W 2>/dev/null) || dir=$PWD  # Use Windows path if on Windows
 
 # Run container and build firmware
 docker run --rm -it $usb_args -v "$dir":/qmk_firmware qmkfm/qmk_firmware \
-	make "$keyboard${keymap:+:$keymap}${target:+:$target}"
+	/bin/bash -c "make git-submodule; make \"$keyboard${keymap:+:$keymap}${target:+:$target}\""