Nicer output for 'docker build'

```bash
$ docker build .
Caching Context 20480/? (n/a)
Step 1 : FROM ubuntu:12.10
 ---> b750fe79269d
Step 2 : RUN apt-get update
 ---> Using cache
 ---> 3e11a97c3f82
Step 3 : RUN apt-get install -q -y znc
 ---> Using cache
 ---> 9dc7a7486e87
Step 4 : ADD . /src
 ---> Running in cd485a64f4e5
 ---> e5c7a600f55f
Step 5 : RUN cd /src && chmod +x zncrun && cp zncrun /usr/local/bin/
 ---> Running in 4364147033ee
 ---> 530281523792
Step 6 : RUN mkdir /.znc && chown irc: /.znc
 ---> Running in 55cbeda6cfea
 ---> 3bc2659ff2cf
Step 7 : EXPOSE 6667
 ---> Running in 1c98533d0fb8
 ---> 9a0c13d994e3
Step 8 : CMD ["zncrun"]
 ---> Running in d0d676825465
 ---> 1131036da4b5
Successfully built 1131036da4b5
```

Note: the "caching context" line is still ugly, but is being fixed in a different PR.