How to connect an external directory or file to a Docker container during its startup?

To connect an external directory or file to a Docker container at startup, the -v (volume) option is used in the docker run command.

Syntax:

docker run -v /path/on/host:/path/in/container image_name

Example of connecting a directory:

docker run -v /home/user/data:/app/data myapp

This mounts the local folder /home/user/data into the container at /app/data.

To connect a single file:

docker run -v /home/user/config.yaml:/app/config.yaml myapp

Thus, the container gains access to the necessary files or folders from the host without needing to copy them into the image.