Creating Docker images

What is a Docker Image?

A Docker image is a template that contains the application code, libraries, dependencies, and configuration required to run a Docker container. It's essentially a snapshot of a filesystem at a particular point in time.

Creating a Docker Image

To create a Docker image, you need to create a Dockerfile which is a text file containing instructions on how to build the image. The Dockerfile specifies the base image, copies files into the image, sets environment variables, and more.

Here's an example Dockerfile for creating an image of a Node.js application:

dockerfile
# Use an official Node.js runtime as our base image
FROM node:14
# Set the working directory to /app
WORKDIR /app
# Copy the contents of the current directory into the container at /app
COPY . /app
# Install any dependencies specified in package.json
RUN npm install
# Make port 3000 available to the world outside this container
EXPOSE 3000
# Define environment variable
ENV NAME World
# Run command when image is created
CMD ["node", "server.js"]

Explanation of Dockerfile instructions:

1. FROM node:14: This line uses an official Node.js runtime as our base image.
2. WORKDIR /app: This sets the working directory in the container to /app.
3. COPY . /app: Copies the contents of the current directory into the container at /app.
4. RUN npm install: Installs any dependencies specified in package.json within the container.
5. EXPOSE 3000: Makes port 3000 available to the world outside this container.
6. ENV NAME World: Sets an environment variable named NAME with value World.
7. CMD ["node", "server.js"]: This line specifies what command to run when a new container is launched from this image.

Building and Running the Docker Image

Once you have created your Dockerfile, save it in a directory along with your project files (in this case, your Node.js application code). Then, navigate to that directory in your terminal and run:

bash
docker build -t my-node-app .

This will create an image named my-node-app. To verify the creation of the image, use:

bash
docker images

After verifying the existence of your image, you can run a container from it using:

bash
docker run -p 3000:3000 my-node-app

This will start a new container from my-node-app and make port 3000 available on the host machine.