Documentation

I know nothing about wasm or emscripten, but am curious. I know next-to-nothing (anymore) about Docker. So, naively working my way through Tools/wasm/README.md I see:

docker run --rm -ti -v $(pwd):/python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3

followed immediately by

mkdir -p builddir/build
pushd builddir/build
../../configure -C
make -j$(nproc)
popd

The docker run command puts me in a docker shell, from which the configure command is pretty worthless. It seems I am supposed to be back at my computer's prompt, but what good is the docker run command at that point? Am I supposed to check out .../python/cpython inside the docker env? I'm clearly missing something and suspect other people new to wasm probably are as well.