Run a command in a running container with docker exec.

# To enter an interactive shell session on an already-running container:
docker exec -it <container_name> </bin/bash>

# To run a command in the background (detached) on a running container:
docker exec -d <container_name> <command>

# To select the working directory for a given command to execute into:
docker exec -it -w <path/to/directory> <container_name> <command>

# To run a command in background on existing container but keep 'stdin' open:
docker exec -i -d <container_name> <command>

# To set an environment variable in a running bash session:
docker exec -it -e <variable_name>=<value> <container_name> </bin/bash>