How to build, run and debug Docker containers

Run the following command to take a look at the container image it pulled from Docker Hub:

docker images

Run the container

docker run hello-world

Finally, look at the running containers by running the following command

docker ps

Build Docker Container

Execute the following command to create and switch into a folder named test

mkdir test && cd test

Create a Dockerfile:

cat > Dockerfile <<EOF
# Use an official Node runtime as the parent image
FROM node:lts
# Set the working directory in the container to /app
WORKDIR /app
# Copy the current directory contents into the container at /app
ADD . /app
# Make the container's port 80 available to the outside world
EXPOSE 80
# Run app.js using node when the container launches
CMD ["node", "app.js"]
EOF

Run the following to create the node application

cat > app.js <<EOF
const http = require('http');
const hostname = '0.0.0.0';
const port = 80;
const server = http.createServer((req, res) => {
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello World\n');
});
server.listen(port, hostname, () => {
    console.log('Server running at http://%s:%s/', hostname, port);
});
process.on('SIGINT', function() {
    console.log('Caught interrupt signal and will exit');
    process.exit();
});
EOF

Note again the “.”, which means current directory so you need to run this command from within the directory that has the Dockerfile

docker build -t node-app:0.1 .

Now, run the following command to look at the images you built

docker images

RUN docker container

docker run -p 4000:80 --name my-app node-app:0.1

Test the server

curl http://localhost:4000

docker stop my-app && docker rm my-app

Now run the following command to start the container in the background

docker run -p 4000:80 --name my-app -d node-app:0.1
docker ps

Debugging Container

Notice the container is running in the output of docker ps. You can look at the logs by executing docker logs [container_id]

docker logs [container_id]
cd test

Edit app.js with a text editor of your choice (for example nano or vim) and replace “Hello World” with another string

cat > app.js <<EOF
const http = require('http');
const hostname = '0.0.0.0';
const port = 80;
const server = http.createServer((req, res) => {
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Welcome to Cloud\n');
});
server.listen(port, hostname, () => {
    console.log('Server running at http://%s:%s/', hostname, port);
});
process.on('SIGINT', function() {
    console.log('Caught interrupt signal and will exit');
    process.exit();
});
EOF

docker build -t node-app:0.2 .

Run another container with the new image version. Notice how we map the host’s port 8080 instead of 80. You can’t use host port 4000 because it’s already in use

docker run -p 8080:80 --name my-app-2 -d node-app:0.2
docker ps

Test the containers

curl http://localhost:8080

And now test the first container you made

curl http://localhost:4000

You can look at the logs of a container using docker logs [container_id]. If you want to follow the log’s output as the container is running, use the -f option

docker logs -f [container_id]

You can use docker exec to do this. Open another terminal (in Cloud Shell, click the + icon) and enter the following command

docker exec -it [container_id] bash

Look at the directory

ls

Exit the Bash session

exit

You can examine a container’s metadata in Docker by using Docker inspect

docker inspect [container_id]

Use — format to inspect specific fields from the returned JSON. For example

docker inspect --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' [container_id]