A Simple Analogy

Docker containers are like shipping containers. They package your application and everything it needs (code, runtime, libraries) into a standardized box. This box runs the same way everywhere—on your laptop, a server, or the cloud.

---

What Is Docker?

Docker is a containerization platform that packages applications and their dependencies into isolated, lightweight containers. Containers are portable, reproducible, and more efficient than virtual machines.

---

Why Use Docker?

• Consistency: "Works on my machine" → "Works everywhere"
• Isolation: Applications don't interfere with each other
• Efficiency: Lightweight compared to full VMs
• Scalability: Easy to deploy multiple instances
• Simplifies DevOps: Single image → multiple environments

---

Images vs. Containers

| Concept | Meaning | |---------|---------| | Image | Blueprint (read-only template) | | Container | Running instance of an image |

---

Basic Workflow

1. Create a Dockerfile

FROM node:18
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

2. Build an Image

docker build -t my-app:1.0 .

3. Run a Container

docker run -p 3000:3000 my-app:1.0

---

Key Docker Commands

docker images                    # List images
docker ps                       # List running containers
docker ps -a                    # List all containers
docker logs <container-id>      # View container logs
docker exec -it <id> bash       # Enter container shell
docker stop <container-id>      # Stop container
docker rm <container-id>        # Remove container

---

Practical Example

# ASP.NET Core API
FROM mcr.microsoft.com/dotnet/sdk:7.0 AS build
WORKDIR /src
COPY ["MyApi.csproj", "."]
RUN dotnet restore "MyApi.csproj"
COPY . .
RUN dotnet build "MyApi.csproj" -c Release -o /app/build

FROM mcr.microsoft.com/dotnet/aspnet:7.0
WORKDIR /app
COPY --from=build /app/build .
EXPOSE 80
ENTRYPOINT ["dotnet", "MyApi.dll"]

Run and check logs:

docker build -t my-api:1.0 .
docker run -p 80:80 my-api:1.0
docker logs <container-id>

---

Related Concepts to Explore

• Docker Compose (multi-container apps)
• Docker Registry and Docker Hub
• Container orchestration (Kubernetes)
• Networking and volumes in Docker

---

Summary

Docker enables developers to package, ship, and run applications reliably across any environment. Master Dockerfile creation and basic commands to get started with containerization and modern DevOps practices.