container_name syntax breaks docker-compose run

[dunk]

When using the container_name syntax added in #1711 "docker-compose run" fails with a message like the following:

Conflict. The name "myservice" is already in use by container bc5601faea86. You have to delete (or rename) that container to be able to reuse that name.

Example docker-compose.yml:

myservice:
    container_name: myservice_name
    image: ubuntu:latest
    command: tail -f /dev/null

Commands run:

docker-compose up -d
docker-compose run --rm myservice /bin/sleep 3

"docker-compose run" is using the same directive to name its container, as can be verified by changing the container_name in docker-compose.yml after starting the service, and running:

docker-compose run -d myservice tail -f /dev/null

I changed the service name to "myservice_name_another", which became the instantiated container's name.

[aanand]

D'oh. Fixing it now.

[dunk]

An otherwise extremely useful feature!

[aanand]

#1755