Fix Docker container creation and error handling

[kvchitrapu]

1. Fixes #102. Print an error if Docker is not reachable. As docker restart tried in every session, server will automatically recover even if docker bounces.
2. Fixed some python module import order in session.py and sandbox.py

Tests

When docker is not running:

INFO:     Started server process [58282]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     ('127.0.0.1', 59155) - "WebSocket /ws" [accepted]
INFO:     connection open
INFO:     Started server process [63469]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     ('127.0.0.1', 60186) - "WebSocket /ws" [accepted]
INFO:     connection open
Failed to stop container: Error while fetching server API version: ('Connection aborted.', PermissionError(13, 'Permission denied'))
Error creating controller. Please check Docker is running using `docker ps`.
Error! Error while fetching server API version: ('Connection aborted.', PermissionError(13, 'Permission denied'))```

When docker is running:

INFO: ('127.0.0.1', 60325) - "WebSocket /ws" [accepted] INFO: connection open INFO: connection closed

============== STEP 0 huggingface/tokenizers: The current process just got forked, after parallelism has already been used. Disabling parallelism to avoid deadlocks... To disable this warning, you can either: - Avoid using tokenizers before the fork if possible - Explicitly set the environment variable TOKENIZERS_PARALLELISM=(true | false)

    ACTION: