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Docker Tutorial for Beginners - Techworld with Nana

What is Docker?

This section was already familar for me from various introductory videos on the topic.

What is a Container?

A container is made of layers of images.

Dividing a container into layers makes pulling diferent versions of the same container much faster, for example.

The difference between image and container is similar to the one between program and process.

Docker vs Virtual Machine

A hosts consists of three layers:

  1. Applications
  2. Kernel
  3. Hardware

While VMs virtualise layers 1 and 2, Docker only virtualises 2. This results in better size, speed, but sacrifices compatibility.

Basic commands

docker run options:

  • -d: detached mode
  • -p<host_port>:<container_port>: Binds the given host and container port
  • --name <name>: Assigns a certain name to a container

Debugging a Container

Docker commands for debugging:

  • logs: See logs of a container
  • exec: Run a command on a container. docker exec -it <container> <shell> gives an access to the shell

Demo Project Overview

Interesting example of CI, from development to deployment with various technologies.

Developing with Containers

I am not really interested the web technologies used in this example, as I am looking for a sysadmin usecase for docker, so a lot of content won't be written about.

Docker Networks

By grouping containers with a network, data can be transfered between only with their names.

The network command is used to create, list... networks. A container can belong to a network by specifying --net on the run command.

Docker Compose

Uses YAML to structure the run commands on a file. Docs here. Important features:

  • Automatically creates a common network for all the services specified in a file.

Usage:

  • Start containers (and crates network): docker-compose -f <file> up.
  • Stops containers (and removes network): docker-compose -f <file> down.

Dockerfile

A blueprint to create docker images. Very strict and simple syntax, documentation here. Example:

FROM <base_image>

ENV <envvar_name>=<env_var_value> \
    ...

RUN <shell_command>
...

COPY <host_path> <container_path>
...

CMD <entrypoint_command>

Images are based on more basic images through Dockerfiles. This is called image layers

An image is created from a Dockerfile with the build command. -t specifies the tag.

Private Docker Repository

This section is also not very interesting for my intended use of Docker for now, so I am skipping it.

Docker Volumes

Docker Volumes compensate the lack of persistency of the Virtual File System of the Container. A Volume is nothing more than a host file (normally a directory) mounted on the Virtual File System.

There are three type of volumes:

  • Host volume: Host and container paths are specified.
  • Anonymous volume: Container path is specified but the host one is automatically generated.
  • Named volume: Same as anonymous but a name is given to the volume.

Volumes can be defined also in Docker Compose.

Anonymous and named volumes are stored in /var/lib/docker/volumes.