Added docker build environment

docker/entrypoint.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+if [[ -z "$QMK_TARGET" ]]; then
+	echo "You must provide a target to build!" 1>&2
+	exit 1
+fi
+
+make $QMK_TARGET
+
+# Move output files to output directory.
+# These are the only extensions qmk can produce,
+# as far as I know.
+mv *.{bin,hex,uf2} /build_output > /dev/null 2>&1