Docker - Docker Basics

What Is Docker?

Deploying and updating software across different operating systems can be cumbersome. Docker simplifies this by providing an isolated environment that runs independently of the host system. It has its own filesystem and network stack, which means that unless explicitly configured, external systems cannot connect to a Docker container.

A bit of history: although containerization has been around since 2010, Docker has emerged as the most popular tool in this space due to its ease of use and flexibility. Kubernetes, a powerful orchestration tool, is often used in conjunction with Docker to manage and scale multiple containers simultaneously.

How Docker Works in a Nutshell: Docker works by emulating the CPU, RAM, and other resources of the host operating system, creating a controlled environment known as a “sandbox.” This sandbox allows software to be installed and run in isolation, ensuring consistent behavior across different environments.

Docker is actually a client-server model.

• Docker client is the command line tool docker...
• Docker Daemon, or dockerd is the background server that manages docker container, images, networks, and storage volumes

Basic Operations

Set up

1
sudo usermod -aG docker $USER

• Adding $USER to the group docker. -a means add, -G means group. By default, running docker requires sudo priviledges. The docker group controls which users can interact with docker,

Stopped Docker Containers

1
2
3
4
5
6
docker ps -a checks all containers, including the stopped ones
docker container prune removes all stopped containers
docker container rm <container-id>

# or
docker rm $(docker ps -a -f status=exited -q)   # -f followed by status

Docker Compose

Amazon Elastic Container Registry (ECR) is a popular place to store Docker containers. First, make sure you have installed AWS CLI. Then, to pull your image:

1
2
aws configure
aws ecr get-login-password --region <REAGION> | docker login --username AWS --password-stdin <ECR_IMAGE_PATH>

To Check what’s in the ECR registry:

1
2
3
# get the registry's name
aws ecr describe-repositories --region <REGION>
aws ecr describe-images --repository-name <REGISTRY_NAME> --region <REGION>

Docker Image Removal

The most vanilla version is docker rmi <IMAGE_SHA_OR_IMAGE_WITH_TAG>

If you want to delete multiple images with the same name but with different tags IMAGENAME:tag1, IMAGENAME:tag2, then you can probably use docker rmi $(docker images 'IMAGENAME' -q).

However, if you want to delete a series of images that are built on top of each other (build stages), then they have a dependency chain. In that case, you can check for the dependencies using docker image --tree and do docker rmi <TOP_IMAGE> ... <BOTTOM_IMAGE>.

Environment Variables

• DEBIAN_FRONTEND: this is an environment variable for Debian-based Systems (like Ubuntu) to control prompts in apt-get. ENV DEBIAN_FRONTEND=noninteractive will assume default answers to all prompts

Docker Commands

• docker kill vs docker stop. docker kill sends a SIGKILL signal to the container, which forcibly kills a container, terminating it immediately without waiting for a graceful shutdown. docker stop sends a SIGTERM signal, which waits for the container to shutdown gracefully.

  • Either command just stops the container process, but the container itself (filesystem, name, etc.) still exists in the Docker’s state.
  • docker run -it --rm --name rico_test simple-robotics-test-image has --rm in it. --rm will respond to only docker stop (graceful exit). Use this command instead: docker rm

Docker Run Args

docker run: this is how to start a docker container. Args that I use quite often are:

• -w ${WORKDIR}: set WORKDIR such that when logging in, one will be in WORKDIR. If there’s /WORKDIR /home/${USER_NAME} it’d work, too.
• /bin/bash -c {COMMAND}: use bash to execute a command upon starting a container.
• --user $(id -u):$(id -g) : running the container with hosts’ UID and GID. So the container user does not have sudo priviledges
  • So you can’t write to system directories like /usr/local/lib. The /usr, /opt, /var directories need sudo priviledges to modify

Common Scenarios and Use Cases

• Enable reverse-i-search in the container:

  • Method 1: inject a .inputrc during container starting time

    1
    2
    3
    4
    5
    6
    7
    8
    ```python
    docker run \
        ...
        ${IMAGE_NAME}:${TAG_NAME} /bin/bash -c "\
    echo 'Creating .inputrc file' && \
    cat <<EOL > /root/.inputrc \"\e[A\": history-search-backward \"\e[B\": history-search-forward EOL
    bash"
    ```
    

  • Method 2: create a .inputrc in image:

    1
    2
    3
    4
    ```
    RUN echo '"\e[A": history-search-backward' >> /home/${USER_NAME}/.inputrc && \
        echo '"\e[B": history-search-forward' >> /home/${USER_NAME}/.inputrc && \
    ```
    

• Previous
  Linux - Operating System
• Next
  VSCode