Docker Quick Ref.

What is containerization ?

A way of packaging applications and its dependencies which can then run on a compatible runtime.

docker

One of the most popular container runtimes. The best way of learning docker and its fundamentals is docker-desktop. Install for your OS and follow the examples. It also comes with a kubernetes cluster.

Dockerfile

Project file that consists of step by step instruction to perform on a base OS image.

# base image
FROM openjdk:11
# steps
COPY app.jar /usr/src/project
WORKDIR /usr/src/project
# command to execute
CMD ["java", "-jar",  "app.jar"]

docker build

Once a dockerfile is written the next step is to build the image. Image is nothing but a container which is not running yet.

docker build -t <repository:tag> <dockerfile_path>

once the build is done you can see the built image using:

docker images
REPOSITORY   TAG   IMAGE ID  CREATED  SIZE

docker run

Easiest way to run the image is:

docker run -p hostPort:containerPort <repository:tag>

and access the application using hostPort. For more options see the docs

runtime logs and image/container cleanup is not in the scope of this quick ref discussion.

docker compose

In most application deployments there are multiple services that run together and communicate with each other. docker compose lets you define those dependencies and make the networking happen.

Here is an example docker-compose YAML file that runs two images, one web application and one database together and lets them address each other internally. Notice the use of db as a dependency and in datasource URL instead of IP or host name. docker compose takes care of passing the relevant environment variables while running the images as per the dependency requirement.

version: "3"

services:
  db:
    image: "postgres"
    ports:
      - "5432:5432"
    environment:
      POSTGRES_DB: testdb
      POSTGRES_USER: testuser
      POSTGRES_PASSWORD: testpass
  app:
    build: .
    ports:
      - "80:8080"
    environment:
      DB_URL: jdbc:postgresql://db/testdb
      DB_USERNAME: testuser
      DB_PASSWORD: testpass
    depends_on:
      - db

docker compose up