Create stand-alone Docker images via entrypoint

[remram44]

Right now, reprounzip run does the work, looking up environment variables/command-lines/... This makes the created Docker images useless by themselves, you need to run them through reprounzip.

By moving some of the run logic from Python to an entrypoint (shell), I can make an image that can be run directly from Docker as well.

Step towards #169

$ docker run -ti --rm reprounzip_image_351oi6lubs help
Image built from reprounzip-docker
Usage: docker run <image> [cmd word [word ...]] [run <R>]
    `cmd ...` changes the command for the next `run` option
    `run <name|number>` runs the specified run
By default, all the runs are executed.
The runs in this image are:
    get_data: python 01_getdata.py
    build_classifier: python 02_classifier.py
    predict: python 03_predict.py
    evaluate: python 04_confusion_matrix.py

$ docker run -ti --rm reprounzip_image_351oi6lubs
Confusion matrix:
[[87  0  0  0  1  0  0  0  0  0]
 [ 0 88  1  0  0  0  0  0  1  1]
 [ 0  0 85  1  0  0  0  0  0  0]
 [ 0  0  0 79  0  3  0  4  5  0]
 [ 0  0  0  0 88  0  0  0  0  4]
 [ 0  0  0  0  0 88  1  0  0  2]
 [ 0  1  0  0  0  0 90  0  0  0]
 [ 0  0  0  0  0  1  0 88  0  0]
 [ 0  0  0  0  0  0  0  0 88  0]
 [ 0  0  0  1  0  1  0  0  0 90]]

[remram44]

Seems to be working. Lots can go wrong here, most notably with quoting/escaping, so testing required.