Show docker-compose down -v removes anonymous volumes from the Dockerfile

dc.yml

version: '2'
services:
  web:
    build: .

Dockerfile

FROM alpine:3.6
RUN mkdir /foo
VOLUME /foo

gistfile1.txt

$ docker volume ls
DRIVER              VOLUME NAME

$ docker-compose up -d
Building web
Step 1/3 : FROM alpine:3.6
 ---> 7328f6f8b418
Step 2/3 : RUN mkdir /foo
 ---> Running in eb658aaf06b8
 ---> cd9da49cdafe
Removing intermediate container eb658aaf06b8
Step 3/3 : VOLUME /foo
 ---> Running in 71008c613b97
 ---> a644a350204b
Removing intermediate container 71008c613b97
Successfully built a644a350204b
Successfully tagged tmp_web:latest
WARNING: Image for service web was built because it did not already exist. To rebuild this image you must use `docker-compose build` or `docker-compose up --build`.
Creating tmp_web_1 ...
Creating tmp_web_1 ... done

$ docker volume ls
DRIVER              VOLUME NAME
local               d699f740385d7656088fcfb9a4f3700dc8ecc555c7de616e06e24f25bb3aca38

$ docker-compose -f /tmp/dc.yml down -v
Removing tmp_web_1 ... done
Removing network tmp_default

$ docker volume ls
DRIVER              VOLUME NAME