Reduce the size of Docker image

1) Add `--no-install-recommends` option to `apt-get install`, this
   avoids lots of (~30MB) unnecessary packages.
2) Add `ca-certificates` package for `wget` fetching stuffs from Amazon
   S3.
3) There is no need to run `apt-get clean` for an image derived from
   official Ubuntu since they already cleaned (see also:
   http://goo.gl/B2SQRB) all the garbages produced by `apt-get`.

Signed-off-by: kfei <kfei@kfei.net>