Improve package build instructions

[josephrocca]

Per this comment by @hoodmane.

Note that I haven't added the run_docker instructions because I don't know how to set the -v flag like I've done here (using the docker run command manually).

What would be the approximate equivalent of this if using run_docker?

git clone https://github.com/pyodide/pyodide
cd pyodide
docker run --rm -it -v $(pwd):/pyodide -w /pyodide pyodide:latest bash
make

(Or is it not necessary to mount a volume? Wondering how else it'd be possible to conveniently get the built files out of the docker container.)

[hoodmane]

Thanks @josephrocca!

[hoodmane]

(Or is it not necessary to mount a volume? Wondering how else it'd be possible to conveniently get the built files out of the docker container.)

Well run_docker mounts the current working directory as /src:

pyodide/run_docker

Line 125 in f3c8445

-v "$PWD":/src \

[josephrocca]

Well run_docker mounts the current working directory as /src:

Thanks, I've added this to the instructions.

[hoodmane]

Thanks @josephrocca!