Docker

Basics

From Basics

Docker Basics

Docker is a containerization platform that allows developers to package applications and their dependencies into lightweight, portable containers. Containers ensure consistent behavior across different environments, from development to production.

What is Docker?

Docker enables you to run applications in isolated environments called containers. Unlike virtual machines, containers share the host OS kernel, making them more efficient and faster to start.

Key Benefits

Consistency: Applications run the same way everywhere
Isolation: Each container runs independently
Portability: Containers work across different platforms
Efficiency: Lower resource overhead than VMs

Core Concepts

Images vs Containers

Component	Description	Example
Image	Read-only template containing application code	`nginx:latest`
Container	Running instance of an image	Active web server
Dockerfile	Text file with instructions to build images	Recipe for creating images

Docker Architecture

graph TB A[Docker Client] --> B[Docker Daemon]; B --> C[Images]; B --> D[Containers]; B --> E[Networks]; B --> F[Volumes]; C --> G[Docker Registry/Hub];

Essential Commands

Image Management

docker pull <image> - Download an image
docker images - List local images
docker build -t <name> . - Build image from Dockerfile

Container Operations

docker run <image> - Create and start a new container
docker ps - List running containers
docker ps -a - List all containers (including stopped)
docker stop <container> - Stop a running container
docker rm <container> - Remove a container

Common Run Options

-d - Run in background (detached mode)
-p 8080:80 - Map host port 8080 to container port 80
-v /host/path:/container/path - Mount volume
--name myapp - Assign custom name

Docker Workflow

sequenceDiagram participant Dev as Developer participant DF as Dockerfile participant Img as Image participant Con as Container Dev->>DF: 1. Write Dockerfile Dev->>Img: 2. docker build Dev->>Con: 3. docker run Con->>Dev: 4. Application running

Dockerfile Basics

A Dockerfile contains step-by-step instructions to build an image:

FROM node:16
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Common Instructions

FROM - Base image to start from
WORKDIR - Set working directory
COPY - Copy files from host to container
RUN - Execute commands during build
EXPOSE - Document which ports the app uses
CMD - Default command when container starts

Best Practices

Security & Efficiency

Use official base images when possible
Keep images small by using minimal base images
Don't run containers as root user
Use .dockerignore to exclude unnecessary files

Development Workflow

Write application code
Create Dockerfile
Build image locally
Test container functionality
Push to registry for deployment

Docker fundamentally changes how applications are packaged and deployed, providing consistency and reliability across the entire software delivery pipeline.

From Bind Mount

Go to text →

https://docs.docker.com/engine/storage/bind-mounts/

Bind Mount: Sharing/Accessing directory that is on host OS.

docker run --mount type=bind,src=<host-path>,dst=<container-path>

Troublehsooting

From Troubleshooting Empty Directory Gotcha

Go to text →

The "Empty Directory" Docker Gotcha on macOS and Windows

The Symptom:

You try to bind-mount a host directory into a Docker container using a command like

docker run --mount type=bind,src=/path/on/host,dst=/path/in/container ...
docker run --volume /path/on/host:/path/in/container ...

The command runs successfully without any errors, but when you inspect the destination directory inside the container, it's completely empty, even though the source directory on your host machine has files.

The Cause:

On macOS and Windows, Docker doesn't run natively. It operates inside a lightweight Linux Virtual Machine (VM) for performance and compatibility. For security reasons, this VM has a "wall" around it and cannot access your computer's entire filesystem by default.

When you ask Docker to mount a directory that is outside of its pre-approved sharing list, it doesn't fail. Instead, it silently creates a new, empty directory within the VM's own filesystem and mounts that into your container. The result is an empty folder, and no error message to tell you why.

The Fix: Granting Docker Access

You must explicitly tell Docker which of your host directories it is allowed to access.

Open Docker Desktop Settings: Click the Docker icon in your menu bar/system tray and select Settings (or Preferences).
Navigate to File Sharing: Go to Resources > File Sharing.
Add Your Directory: You'll see a list of paths Docker can already access (e.g., /Users, /tmp). Click the + button and add the parent directory you want to mount from (e.g., /Users/yourname/projects).
Apply and Restart: Click the Apply & Restart button. Docker will restart its VM with the new permissions.

Once restarted, your docker run command will work as expected, and the directory will be correctly mounted with all its contents.

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