The latter needs to be run once to build the Docker image, while the former you run each time you want to enter your (unified) build environment. To make it even easier (your definition of "easier" might be different than mine; what I mean here is that you can get away without having to mount an installation media image in qemu and manually install a Linux distro to your qemu image), you can take advantage of a great tool called debos, which builds a complete image for you to be run in qemu.