Docker

Last Updated: 2022-10-13

What is Docker

When we talk about Docker, usually we are referring to Docker Engine, which consists of

the Docker daemon (dockerd)
a REST API that specifies interfaces for interacting with the daemon
a command line interface (CLI) client (docker) that talks to the daemon (through the REST API wrapper), e.g. docker run <image>, docker image ls.

Because Docker operates at the OS level, it can still be run inside a VM!

2 most important APIs: Images and Container APIs

Get Info

$ docker info

Docker Root Dir: e.g. /var/lib/docker/ where the images are stored.

Go inside the container

$ docker exec -it <container-name> bash

Start a Ubuntu 20.04

$ docker run -it --entrypoint "/bin/bash" ubuntu:20.04

credHelper

credHelpers can be set in ~/.docker/config.json

"credHelpers": {
    "gcr.io": "gcloud"
}

It means any image pull from gcr.io will use the binary docker-credential-gcloud to get the username and secret. (The binary = docker-credential- + suffix)

echo gcr.io | docker-credential-gcloud get

To login:

docker login 10.200.0.1 -u admin -p ${REGISTRY_PASSWORD}

Docker Compose vs Docker Stack

docker-compose: a tool for defining and running multi-container Docker applications; a separate tool built in Python, internally uses the Docker API to bring up containers according to the specification
docker stack: built-in docker CLI, no additional packages needed; written in Go; (successor of docker-compose?)
both works with docker-compose.yml, however docker stack only works with version 3.

Docker for Mac

The Docker for Mac application does not use docker-machine to provision that VM; but rather creates and manages it directly.

CMD vs ENTRYPOINT

Both CMD and ENTRYPOINT instructions define what command gets executed when running a container. There are few rules that describe their co-operation.

Dockerfile should specify at least one of CMD or ENTRYPOINT commands.
ENTRYPOINT should be defined when using the container as an executable.
CMD should be used as a way of defining default arguments for an ENTRYPOINT command or for executing an ad-hoc command in a container.
CMD will be overridden when running the container with alternative arguments.

Run Docker Engine

$ sudo dockerd

Network

List networks:

$ docker network ls

DRIVER:

null: the container does not have external network interfaces, only a local loopback interface.
host: the container is attached to the host's network, the configs inside the container matches the configs outside the container.
bridge: containers connected to the same bridge network can communicate; containers on different bridge networks cannot communicate directly with each other.

To get more details:

$ docker network inspect bridge

Dockerize a Node app

Use this Dockerfile template:

# node
FROM node:12-slim

# the path inside the container
WORKDIR /usr/src/app

# copy the package.json and package-lock.json, and install dependencies
COPY package*.json ./
RUN npm install

# copy all the source code
COPY . .

# port
EXPOSE 8080

# run the command inside the container
CMD [ "node", "app.js" ]

And add a .dockerignore file:

Dockerfile
.dockerignore
node_modules
npm-debug.log