N/A

that would depend on your needs

for the image, it's just the working directory as the command says

and it will be the output of the pwd command if you execute that in the image

where you mount the volume is completely independent and is defined through absolute paths

the only caveat would be if your app's scope is the same than the working directory and you decide to attach your volume outside of it, so you wouldn't have access to the volume

Using Nixpack was the better route.