Docker by Example: Web app containers
Open up the 2-node-app folder (in your text editor and in your terminal)
Much of the time you work with Docker, it will be in a web development context. It can make the job easier in a lot of ways, so let's see what that looks like.
Open up the Dockerfile in this folder to see what we're doing here. First, you see that we're working with a NodeJS application. The first step is to set our working directory inside the image. All the rest of the commands will do work based on that directory.
FROM node:22
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]
Next, we copy in the package.json file, then run npm install. If you know NodeJS, you know that means we're installing the dependencies for an application. (Remember, this is all happening inside the image. Nothing is changing on your computer's hard drive). Next we copy the contents of the current directory (the first . on line 9) into the image's working directory (the second . on line 9).
Then we EXPOSE a port - this is an important concept. Within the Dockerfile, we have to tell the container to open up port 3000 (TCP by default) to the outside world, allowing traffic to pass in from outside the container.
Finally, we have CMD ["npm", "start"]. CMD tells our container what command to execute when we run the container.
Build and run this image
docker build -t web-app:v1 .
docker run web-app:v1This should start up the application, and you'll see a message in your terminal saying that the application is listening on http://localhost:3000. Visit that page in your browser. It didn't work! Press CTRL+C 3 times in your terminal to stop the docker process from running.
Why didn't it work? Because the EXPOSE command in the Dockerfile only tells the image to expose the port. To really use it, you have to also publish the port with the -p argument to the docker run command.
Run the following command:
docker run -p 3000:3000 web-app:v1Now you can visit http://localhost:3000 and see the application in action. Running the command in this way creates a mapping between the external (to docker) port 3000 (the first 3000), and the internal, exposed port 3000 (the second 3000). It publishes the port to your operating system so that you can connect from applications besides Docker. Remember: If you want to connect to applications running in Docker from outside Docker, you need to both EXPOSE and publish (-p) the ports.
Next up
Professor S