📘 Docker Summary Guide

 📘 Docker Summary Guide

I. What is Docker?

Docker is a platform for developing, shipping, and running applications in lightweight, portable containers. Containers package code and dependencies together so applications run reliably across different environments.

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II. Core Concepts

1. Image
   A lightweight, standalone, immutable file that includes the application and all its dependencies.

2. Container
   A runnable instance of an image. Containers are isolated from the host and each other.

3. Dockerfile
   A script used to build Docker images. It contains instructions (like FROM, RUN, COPY, etc.).

4. Docker Engine
   The core client-server application:

   • Docker Daemon (dockerd): Runs in the background.

   • Docker CLI (docker): Command-line interface to interact with Docker.

5. Docker Hub / Registry
   Central repositories where Docker images are stored and shared.

   • Docker Hub is the default public registry.

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III. Docker Architecture

1. Docker Client
   The user interface to Docker. Sends commands to the daemon (via REST API).

2. Docker Daemon
   Manages Docker objects (images, containers, networks, volumes). Listens on a socket.

3. Docker Objects

   • Images

   • Containers

   • Networks

   • Volumes

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IV. Dockerfile Instructions (Key Commands)

Instruction	Description
FROM	Base image to start with.
RUN	Executes command during image build.
COPY / ADD	Copy files into the image.
CMD	Default command when a container runs.
ENTRYPOINT	Sets the main executable.
EXPOSE	Documents port(s) the container listens on.
ENV	Sets environment variables.
WORKDIR	Sets the working directory inside the container.

V. Container Lifecycle

1. Create
   Using docker create or docker run.

2. Start / Stop
   Use docker start / docker stop.

3. Pause / Unpause
   Temporarily suspend or resume.

4. Restart / Remove
   Manage lifecycle and cleanup.

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VI. Docker Networking

1. Bridge Network (default)
   Containers get an IP on a private network; NAT used for external access.

2. Host Network
   Container shares the host network.

3. Overlay Network
   Used in Docker Swarm for multi-host communication.

4. None
   No network access for the container.

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VII. Docker Storage

1. Volumes

   • Managed by Docker. Persistent storage.

   • Shared between containers.

2. Bind Mounts

   • Mount a file/folder from the host into the container.

3. tmpfs Mounts

   • Temporary filesystems stored in memory.

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VIII. Docker Compose

Docker Compose is a tool to define and run multi-container applications using a docker-compose.yml file.

• Define services, networks, volumes.

• Use docker-compose up to start everything.

• Ideal for local development and testing environments.

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IX. Docker Swarm (Optional: Native Orchestration)

Docker Swarm is Docker's built-in container orchestration tool.

• Turns multiple Docker engines into a single cluster.

• Manages container deployment, scaling, load-balancing.

• Use docker swarm init to start a swarm.

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X. Docker Best Practices

1. Keep images small using minimal base images (alpine).

2. Use .dockerignore to exclude files from builds.

3. Minimize layers in Dockerfile.

4. Avoid running containers as root inside.

5. Use multistage builds for cleaner production images.