Document the container environment variables semantics on mesos.

Environment variables might be overwritten on mesos (task env, image env, agent env, etc). We should document how env var could be overwritten.

Also, we need to document the mesos-injected environment variables (like `MESOS_SANDBOX`), and how are they different in different executors:

• What are the mesos-injected environment variables for executor containers
• What are the mesos-injected environment variables for container tasks via default executor
• What are the mesos-injected environment variables for old-style command tasks