Docker workshop

Some introductory usage examples for the Docker workshop.

Contents:

Create/edit Dockerfile
Build Docker image
Docker hub login
Push image
Start and stop a container
Using docker-compose
Deleting stuff
Containers, data and volumes
Build with Maven

Questions to test your knowledge start with a Q.

Dockerfile

As an example, we want to build a Docker image with a Postgres database. A Dockerfile can be found in the directory dockerfile.

Q: where does the base image come from? What does the ENV command? Why multiple lines with \ ?

Build Docker image

To build a new image from the Dockerfile, run the following command:

cd dockerfile
docker build -t myimage .

Q: what does the -t parameter? What is the . for?

This builds a new image with the name myimage and tag latest (the default tag). The new image can be found with

docker images

Hint: you can filter the output with docker images myima* for example.

Docker hub login

Per default, Docker images are pulled from the central Docker hub (https://hub.docker.com/). For example, get the latest Alpine Linux with

docker pull alpine

Q: how to search for Docker images? How to get a specific version?

To access our private Docker hub (or registry), a login is required:

docker login hub.1worldsync.de

If you just want to download images, the 'readonly' user is sufficient, but if you want to push (upload) the images you have built you have to login with a user with write access. Use the user 'docker' in this case. Passwords are provided privately :-)

Q: after a successful login the credentials are stored. Where?

Tag and Push image

To publish a previously built Docker image, it has to be tagged and pushed to the registry.

Tag the image with your Linux username so we can all push our own image. Then tag it with the registry and push the image:

docker tag myimage demo-<My_Userid>/myimage
docker tag demo-<My_Userid>/myimage hub.1worldsync.de/demo-<My_Userid>/myimage
docker push hub.1worldsync.de/demo-<My_Userid>/myimage

Q: does every tag get a new image id? How to check?

Start and Stop a container

The new image can be used to start a new container. Use the docker run command!

Q: on which port does the Postgres database run? Try to connect! How to stop the container?

A running container can be stopped with the docker stop command.

Q: does the container still exist after the stop command?

Stop the container and start it again, but this time with a given name, e.g. pgsql.

Using Docker-Compose

It is easier to run a container with docker-compose. A docker-compose.ymlfile is required. To start the container using docker-compose, use

docker-compose up -d

Check the status with docker ps or docker-compose ps and check the logs.

Q: on which port is the Postgres database running? exposeand ports - where's the difference? How to run the database on port 9999?

To stop and remove the container that was started with docker-compose use

docker-compose down

Q: what's the difference of docker-compose stop and docker-compose down?

Delete images and containers

Docker images are removed with the docker rmicommand, containers are removed with docker rm.

Add some additional tags for your image myimgage with docker tag. Then delete them again, use the tag and the image id as parameter.

Q: how to delete a tag and how is that different from deleting an image?

Containers, data and volumes

After a container has been started, the data that is created is stored in the container filesystem. To store data outside of the container, host directories can be mounted or Docker volumes, a kind of data container, can be used. Volumes are the preferred solution!

We want to create a file in the container. This can be done by running a shell in the container with the 'exec' command:

docker exec -ti myimage ash
touch /test.txt
exit

Q: now stop the container - does the file still exist? Start the container again and check it! Stop and remove the container, then start it again: does the file still exist?

The Postgres image uses a volume to store the data of the database. You can find the configuration in the docker-compose.yml file. A container directory is mounted to the volume.

Q: where is the volume?

Do it with Maven

Go to the directory maven-project for an example project that builds the same image, but with Maven. We use the dockerfile-maven-plugin from Spotify to build images in our software. Do not confuse it with the docker-maven-plugin!

To build the image run