Docker cheatsheet

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Dockerfile

FROM node:latest
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build
EXPOSE 3000
CMD ["npm", "start"]

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Build an image and tag it

docker build -t myapp:1.0 .

. at the end specifies current directory -t or --tag option allows us to name and tag the image in name:tag format Note, that if you don't provide a tag it will be defaulted to latest, which might be something we don't want

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List images

To see all local images, run:

docker image ls

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Run image

docker run --name web myapp:1.0

Note how you can label containers using --name or -n option.

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Stop running container

# Stop container with SIGTERM
docker stop web

# Stop container with SIGKILL
docker kill web

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Stop all running containers

docker stop $(docker ps -q)

docker ps command will list all running containers and -q option will only show the IDs for those containers

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Delete image

# docker rmi is an alias of docker image rm
docker rmi myapp:1.0

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Create docker-compose.yml file and run

version: '3.18'
services:
  app:
    image: myapp:1.0
    env_file:
      - .env
    ports:
      - 80:3000

Note that port configuration in docker-compose.yml will overwrite one in Dockerfile Make sure you have .env and node_modules in .dockerignore file.

To run the app, use:

docker-compose up -d app

-d option will run container in detached mode

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Monitor your running app

To see running containers:

docker ps

To see logs from apps within container:

docker logs -f web

-f or --follow option will follow all incomming logs

To see the list of last 100 logs:

docker logs --tail 100 web

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Setup persistence volume

Docker containers should be ready to be terminated and re-built any moment. Data we're saving to file system within image will be lost in the case of such reload. To fix this issue we need to setup persistent volumes. To add volumes, we need to modify docker-compose.yml

version: '3.18'
services:
  app:
    build: .
    env_file:
      - .env
    ports:
      - 80:3000
    volumes:
      - appdata: /appdata
volumes:
  appdata:

Under app, we need to specify what directory inside image will be served as persistent volume and under volumes we need to declare our volumes. All the data saved in /appdata will persist even in the case of image reload.

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Bind ports in order to export and access services

There are three ways to expose ports with Docker:

1. From Dockerfile using EXPOSE <hostport>
2. From docker-compose.yml under -ports
3. While running container: docker run -p <hostport>:<dockerport> <image>

Note the host port comes first - that's the port we're going to access app from the host machine. So if our Node/Express app is listening to port 3000 and we're binding ports with 80:3000, we can access the app from http://localhost.

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Restart containers to ensure high availability

In stand-alone container:

docker run --restart always --name my_container image_tag

In docker-compose.yml:

version: '3.18'
services:
  webapp:
    image: image_name
    restart: always

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Use environment variables

version: '3.18'
services:
  webapp:
    image: image_name
    restart: always
    env_file:
      - .env

Do not forget to add .env file to both .gitignore and .dockerignore files.

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Pipe logs from file to stdout

In Dockerfile:

RUN ln -sf /dev/stdout ./logs/debug.log

This command will create a symlink (symbolic link -s) from our debug.log file we write logs to to stdout. Note that this will NOT write logs to a file which will not use disk space.