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README.md

GSP055 —— Introduction to Docker

Table of Contents （🔎 Click to expand／collapse）

Overview

Docker is an open platform for developing, shipping, and running applications. With Docker, you can separate your applications from your infrastructure and treat your infrastructure like a managed application. Docker helps you ship code faster, test faster, deploy faster, and shorten the cycle between writing code and running code.

Docker does this by combining kernel containerization features with workflows and tooling that helps you manage and deploy your applications.

Docker containers can be directly used in Kubernetes, which allows them to be run in the Kubernetes Engine with ease.

Basic Commands

# run a hello world container
$ docker run <image-name>     # the container name would be generated randomly
$ docker run <image-name> --name [container-name]

# check the container image pulled from Docker Hub
$ docker images

# check the containers
$ docker ps                   # running containers
$ docker ps -a                # all containers
  • Command docker run <image-name> makes docker daemon searching for the given image. It will find the image locally or pulled the image from a public registry called Docker Hub.
  • Command docker images shows the docker images pulled from the Docker Hub public registry with the unique Image ID (in SHA256 format).
  • Command docker ps shows containers with the Container ID (a UUID generated by Docker to identify the container) and more metadata about the run.

Build Images

We need the file named Dockerfile which instructs the Docker daemon on how to build our Docker Image. For example:

FROM ubuntu:18.04       # creates a layer from the ubuntu:18.04 Docker image
COPY . /app             # adds files from your Docker client’s current directory
RUN make /app           # builds your application with make
CMD python /app/app.py  # specifies what command to run within the container

Let's build the image with docker build command and --tag option. The tag is highly recommended when building Docker images for distinguishing.

# build image with tag
$ docker build --tag <image-name>:<image-tag> .

Run Containers

# run containers based on the built image
$ docker run --publish <container-port>:<host-port> --name <container-name> <image-name>:<image-tag>

# stop and remove containers
$ docker stop <container-name> && docker rm <container-name>
  • Command docker run runs the containers based on the given image
    • The --name flag allows us to name the container if we like.
    • The --publish (-p) flag with <host-port>:<container-port> instructs Docker to map the host's port to the container's port.
    • The --deatch (-d) flag runs containers in the background (not tied to the terminal's session)
  • Command docker stop and docker rm are used to stop and remove the containers.
  • Command docker stop and docker rm are used to stop and remove the containers.

Debug with Containers

# check the logs of containers
$ docker logs <container-id>

# start an interactive Bash session inside the running container
$ docker exec --interactive --tty <container-id> bash

# examine a container's metadata in Docker
$ docker inspect <container-id>
  • Command docker logs shows the logs of containers.
    • The --follow (-f) flag follow the log's output as the container is running.
  • Command docker exec runs a command in running containers.
    • The --interactive (-i) flag keeps STDIN open even if not attached.
    • The --tty (-t) flag allocates a pseudo-TTY.
    • Notice bash runs in the WORKDIR directory specified in the Dockerfile.
  • Command docker inspect returns low-level information on Docker objects.
    • The --format (-f) flag formats the output using the given template.

Publish Images

We can publish the Docker Images we built to the registries such as Docker Hub, Google Container Registry (gcr) or even a self-hosted one.

# check config of project for finding the project-id
$ gcloud config list project

# tag the images with a registry name (needed for gcr)
$ docker tag node-app:0.2 gcr.io/<project-id>/<image-name>:<image-tag>

# push the image to Google Container Registry (gcr)
$ docker push gcr.io/<project-id>/<image-name>:<image-tag>
  • Command docker push pushes an image or a repository to the registry.
    • Google Cloud Platform (GCP) use Google Container Registry (gcr) for storing, managing, and securing our Docker images.
    • We need to tag the images in format [hostname]/[project-id]/[image]:[tag].
  • How to check the image exists in Google Container Registry (gcr)?
    • [Option 1] Navigate to Navigation Menu > Container Registry.
    • [Option 2] Visit with the url http://gcr.io/<project-id>/<image-name>.

References