Dear Maintainer,

`piuparts --docker-image $img` fails if the image is not yet available
and needs to be pulled from a repote docker image repository first. In
that case the output of `docker run` contains progress information in
addition to the container ID.

The 1. attached patch fixed this by using `--cidfile`, which forces
`docker run` to write the generated container ID into that named file,
from where it is read afterwards.

The 2. patch simplifies getting the merged directory path from `docker
inspect`.

The 3. patch is optional and helps with debugging by always using log
level `DUMP` for the given log file and using `--log-level` only for
logging to STDOUT.

<https://docs.docker.com/engine/reference/run/#pid-equivalent>