Volume when is create in end or on middel of execution?

Hi i am wondering when the docker create the volume ?

why i ask that question :
because i had a docker where nodejs have a file called app (generated in host with webstorm and i have all directories views models bin …) . and package.json

i start to use dockerfile and docker-compose.yml to create my env docker but when i try
build: .
- "./app:/src/app"
- "3030:3000"
i found the same name if app inside docker but if i follow the steps execution

of dockerfile :

FROM node:0.10.38

RUN mkdir /src

RUN npm install express-generator -g


ADD app/package.json /src/package.json

RUN npm install


CMD node app/bin/www

i dont understand where he create the volume call app under /src
thanks for advance