Use argument --rm

Each time you run `docker run` a new container is created
```bash
$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
$ docker run -ti -v `pwd`:/srv marmelab/composer-hhvm bash -c 'hhvm /usr/local/bin/composer --version'
Composer version a8adbfeb9fc7861deade782938222714168a22a8 2014-09-05 16:28:50
$ docker run -ti -v `pwd`:/srv marmelab/composer-hhvm bash -c 'hhvm /usr/local/bin/composer --version'
Composer version a8adbfeb9fc7861deade782938222714168a22a8 2014-09-05 16:28:50
$ docker ps -a
CONTAINER ID        IMAGE                           COMMAND                CREATED             STATUS                     PORTS               NAMES
404daa4bc9d1        marmelab/composer-hhvm:latest   bash -c 'hhvm /usr/l   4 seconds ago       Exited (0) 2 seconds ago                       hungry_euclid       
04e9a1345095        marmelab/composer-hhvm:latest   bash -c 'hhvm /usr/l   7 seconds ago       Exited (0) 5 seconds ago                       hungry_carson       
```

On "one shot" container like this one, adding the argument `--rm` delete this temporary container

```bash
$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
$ docker run -ti --rm -v `pwd`:/srv marmelab/composer-hhvm bash -c 'hhvm /usr/local/bin/composer --version'
Composer version a8adbfeb9fc7861deade782938222714168a22a8 2014-09-05 16:28:50
$ docker run -ti --rm -v `pwd`:/srv marmelab/composer-hhvm bash -c 'hhvm /usr/local/bin/composer --version'
Composer version a8adbfeb9fc7861deade782938222714168a22a8 2014-09-05 16:28:50
$ docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
```

see http://docs.docker.com/reference/run/#clean-up-rm
This outputs a warning in my version of docker (1.2.0). The correct syntax seems to be `--rm=true` (see https://github.com/marmelab/make-docker-command/pull/1).