Gracefully stopping tfc-agent in Docker with SIGINT

Graceful shutdown of the tfc-agent in Docker allowing the current Terraform run to finish successfully.

Procedure

Run the following command when stopping the tfc-agent in Docker to prevent the container from exiting while the run(plan or apply) is still in progress.
```
# Replace tfc_agent with your container name
docker kill --signal SIGINT tfc_agent
```