Docker Container Execution & Config 🐳

The Shipping Container Analogy

Imagine you’re running a magical shipping company. Each container is like a small box that carries everything needed for a specific job—tools, supplies, and instructions. Your job? Learn how to launch these containers, name them, connect them to the outside world, and give them the resources they need!

🚀 Running Containers

What Does “Running a Container” Mean?

Think of it like this: You have a recipe (Docker image), and now you want to cook the dish (run the container).

docker run nginx

This command:

1. Finds the nginx image (the recipe)
2. Creates a new container from it
3. Starts it running!

The Basic Command

docker run [OPTIONS] IMAGE [COMMAND]

Real Example:

docker run hello-world

This runs a tiny container that says “Hello from Docker!” and exits.

🏷️ Container Naming

Why Name Your Containers?

Imagine having 100 boxes in a warehouse with no labels. Chaos! Docker gives containers random names like quirky_einstein, but you can choose your own.

docker run --name my-web-server nginx

Now instead of remembering a7f3b2c1d4e5, you just say my-web-server!

Rules for Names

• Letters, numbers, underscores, hyphens allowed
• Must be unique (no two containers with same name)
• Makes your life much easier

# Stop by name
docker stop my-web-server

# View logs by name
docker logs my-web-server

🎭 Detached vs Interactive Modes

Two Ways to Run a Container

Interactive Mode (-it): Like having a live phone call

docker run -it ubuntu bash

• -i = Keep input open (listen to you)
• -t = Give you a terminal (so you can type)
• You’re INSIDE the container, typing commands!

Detached Mode (-d): Like sending a postcard

docker run -d nginx

• Container runs in the background
• You get your terminal back immediately
• Perfect for servers that run 24/7

When to Use Each?

🌉 Container Port Mapping

The Bridge Between Worlds

Your container is like a house on a private island. Port mapping builds a bridge so visitors (network traffic) can reach it!

docker run -d -p 8080:80 nginx

This means:

• 8080 = Door on YOUR computer
• 80 = Door inside the container
• Traffic to localhost:8080 goes to container’s port 80

graph TD
    A[Your Browser] -->|localhost:8080| B[Docker Host]
    B -->|Port Mapping| C[Container :80]
    C --> D[Nginx Running]

Multiple Port Mappings

docker run -d \
  -p 8080:80 \
  -p 8443:443 \
  nginx

Now both HTTP (8080) and HTTPS (8443) work!

🌿 Environment Variables

Passing Secrets & Settings

Environment variables are like sticky notes you attach to your container with instructions.

docker run -e DATABASE_URL="mysql://db:3306" \
           -e APP_MODE="production" \
           my-app

Why Use Them?

• Passwords (don’t hardcode!)
• Configuration (dev vs production)
• API keys and secrets

Using an Env File

Too many variables? Put them in a file!

# .env file
DATABASE_URL=mysql://db:3306
APP_MODE=production
SECRET_KEY=super-secret-123

docker run --env-file .env my-app

⚖️ Environment Variable Precedence

Who Wins When Values Conflict?

Imagine three people giving you instructions. Who do you listen to?

Priority Order (Highest to Lowest):

1. Command line -e (Boss says directly)
2. --env-file (Written instructions)
3. Dockerfile ENV (Default instructions)

# Dockerfile says: APP_MODE=development
# .env says: APP_MODE=staging
# Command says: -e APP_MODE=production

# Result: production WINS! 🏆

graph TD
    A["-e flag"] -->|Highest Priority| D[Final Value]
    B["--env-file"] -->|Medium Priority| D
    C["Dockerfile ENV"] -->|Lowest Priority| D

🔄 Container Restart Policies

What Happens When Things Crash?

Containers can stop unexpectedly. Restart policies tell Docker: “What should I do next?”

Examples

# Always keep running (perfect for servers)
docker run -d --restart=always nginx

# Restart on crashes, max 5 attempts
docker run -d --restart=on-failure:5 my-app

# Restart unless manually stopped
docker run -d --restart=unless-stopped redis

When to Use Each?

• Production servers: always or unless-stopped
• Development: no (you want to see crashes)
• Flaky apps: on-failure:3 (limit retries)

📊 Container Resource Limits

Don’t Let Containers Be Greedy!

Without limits, one container could eat all your computer’s memory and CPU. That’s bad!

Memory Limits

# Max 512 megabytes of RAM
docker run -m 512m nginx

# Max 2 gigabytes
docker run --memory=2g my-database

If the container tries to use more, Docker stops it (or kills it).

CPU Limits

# Use only 1.5 CPU cores max
docker run --cpus=1.5 my-app

# Use 50% of one CPU core
docker run --cpus=0.5 lightweight-app

Combined Example

docker run -d \
  --name production-api \
  --memory=1g \
  --cpus=2 \
  --restart=always \
  my-api:latest

This container:

• Gets max 1GB RAM
• Gets max 2 CPU cores
• Always restarts if it stops

🎯 Putting It All Together

Here’s a real-world example combining everything:

docker run -d \
  --name my-web-app \
  -p 3000:3000 \
  -e NODE_ENV=production \
  -e DATABASE_URL=postgres://db:5432 \
  --restart=unless-stopped \
  --memory=512m \
  --cpus=1 \
  my-node-app:v2

What this does:

1. ✅ Runs in background (-d)
2. ✅ Named my-web-app
3. ✅ Maps port 3000
4. ✅ Sets environment variables
5. ✅ Restarts automatically
6. ✅ Limited to 512MB RAM and 1 CPU

🌟 Key Takeaways

You’re now ready to launch, configure, and control Docker containers like a pro! 🚀