Fix detecting Docker container ID when pulling image the first time

We (Univention GmbH) are running piuparts using our internal docker infrastructure for several years by now, which is working very well. Two patches were needed to support same corner cases in the current code:

• if the used image is not yet available locally, it will get pulled. This prints extra lines, which breaks the current code to extract the container ID from the output.

• I had some issues getting the working directory for inspection, but no longer remember the details. I changes the code to only ask for .GraphDriver.Data.MergedDir instead of using JSON to extract that small information from the larger dataset.

• Also log directory name to help further debugging.

• debian/changelog is included

• We're running this for years, so well tested.