Document known differences between Docker and Mesos containerizers

To help developers who create applications which may be run using both the Docker and Mesos containerizers, we should document all known differences from a developer's perspective.

For example:

• By default, Docker starts the task process in / while Mesos containerizer starts the task process in the Mesos task sandbox.
• When command.shell is false, Mesos containerizer launches the command via execvp while Docker passes the command as arguments to the Docker CLI. One difference this causes is that $@ will be empty when the Mesos containerizer is used and the executable is a bash script.