Automatically remove container leftovers

Leverage the `-rm` flag in `docker run` to automatically clean up filesystem read/write layers and other container files.  Container leftovers are now cleaned up automatically when performing a `deis scale web=0`:

```
root@ip-10-250-15-201:/var/lib/docker/containers# docker ps -a
ID                  IMAGE                   COMMAND                CREATED             STATUS              PORTS
920cdeb80690        deis/buildstep:latest   /bin/sh -c export HO   5 minutes ago       Up 5 minutes        10008->10008        
7e24e8c0a448        deis/buildstep:latest   /bin/sh -c export HO   5 minutes ago       Up 5 minutes        10005->10005        
825f7a294d78        deis/buildstep:latest   /bin/sh -c export HO   5 minutes ago       Up 5 minutes        10004->10004 
root@ip-10-250-15-201:/var/lib/docker/containers# ls -l
total 12
drwx------ 4 root root 4096 Dec  4 18:43 7e24e8c0a448b62fa222b6c8bfa4df900eb5935330b20edbac59aa71a3dfebed
drwx------ 4 root root 4096 Dec  4 18:43 825f7a294d78063ce67ba5a8bc6a749ccaf107ea028d1cdb1662f48f3e2ef5e4
drwx------ 4 root root 4096 Dec  4 18:43 920cdeb806908aa2a7db360167db8f4e75bb9208719eb36bb97d33fd420bf64f     
root@ip-10-250-15-201:/var/lib/docker/containers# docker ps -a
ID                  IMAGE               COMMAND             CREATED             STATUS              PORTS
root@ip-10-250-15-201:/var/lib/docker/containers# 
```