[analyzer] SATest: Add an easy option to connect to docker for debugging

Summary:
Docker on its own has a pretty convenient way to run shell.
This method, however, requires target container to be currently running,
which is not a usual scenario for the test system.  For this purpose,
it is better to have a simple way to run the container, shell it, and
clean up at the end of it all.  New option `--shell` does exactly this.

Differential Revision: https://reviews.llvm.org/D81598