What are Docker Images and Containers?

A Docker image is a blueprint or template for a container. It contains the code, runtime, tools, and libraries needed to run an application. Think of it as a snapshot of your application and its dependencies.

A Docker container, on the other hand, is a running instance of an image. It's the actual executable unit of software. You can create multiple containers from a single image.

Creating Docker Images

You can create Docker images using a Dockerfile or by using an existing, pre-built image. A Dockerfile is a text document that contains instructions for building a Docker image.

Key Dockerfile commands include:

• FROM: Specifies the base image to use.
• WORKDIR: Sets the working directory inside the container.
• COPY: Copies files from the host machine to the container.
• RUN: Executes commands inside the container.
• EXPOSE: Exposes a port.
• CMD: Specifies the default command to run when the container starts.

To build a Docker image, you use the docker build command. The basic syntax is docker build -t <image_name> .. The -t flag is used to tag (name) your image.

Essential Docker Commands

Here are some essential Docker commands:

• docker ps: Lists running containers.
• docker ps -a: Lists all containers (running and stopped).
• docker stop <container_id>: Stops a running container.
• docker rm <container_id>: Removes a container.
• docker run: Runs a container from an image.
• docker run -it <image_name> /bin/bash: Runs a container in interactive mode with a shell.
• docker run -d: Runs a container in detached mode (in the background).
• docker run -p <host_port>:<container_port>: Publishes a container's port to the host.
• docker tag <image_name>: Tags an image.
• docker system df: Shows disk usage.
• docker logs <container_id>: Retrieves logs from a container.

Managing Data with Volumes

Docker volumes are used to persist data generated by containers. Volumes are stored on the host machine and are mounted into containers. This means that data in volumes persists even if the container is stopped or removed. Containers can both read and write data to volumes.

There are three types of volumes:

• Anonymous Volumes: These volumes don't have a name and are managed by Docker. They are created specifically for a single container.
• Named Volumes: These volumes are given a name and are also managed by Docker. They are suitable for persistent data that doesn't need to be edited directly.
• Bind Mounts: With bind mounts, you specify a folder or path on the host machine that is mounted into the container. They are managed by the developer and are great for persistent, editable data like source code.

Key differences between Volume types

• Anonymous volumes are tied to the lifecycle of a container, and unless the -rm flag is used, they persist. They cannot be shared across containers and can be reused even on some image.
• Named volumes are not tied to a specific container, survive container shutdown/restart, and can be shared and re-used across containers.
• Bind mounts are tied to the host file system, survive container shutdown/restart, and can be shared and re-used across containers.

You can also mount a folder as read-only.

Arguments and Environment Variables

Docker supports build-time arguments (ARG) and runtime environment variables (ENV). ARG variables are available during the image build process, while ENV variables are available both in the Dockerfile and in running containers.

In a Dockerfile, you can add environment variables using the ENV instruction.

This introduction provides a solid foundation for working with Docker. Docker simplifies the process of building, shipping, and running applications, making it an essential tool for modern software development.