readme: add missing ./setup to Docker instructions

Author: cirosantilli

README.adoc

@@ -726,6 +726,7 @@ For example, to do a <<qemu-buildroot-setup>> inside Docker, run:
 
 ....
 sudo apt-get install docker
+./setup
 ./run-docker create && \
 ./run-docker sh -- ./build --download-dependencies qemu-buildroot
 ./run-docker