Docker

General

• Docker Hub
• Docker Docs
• Docker command reference
• Docker networking
• Dockerfile reference
• Images:
  • Alpine Linux
  • Fedora Linux

Install/start docker

dnf install docker
systemctl start docker

Control images:

docker pull <name>
docker image ls
docker image rm <image>

Control containers:

• run creates a container from an image
• capture container stdin with -it
• map host filesystem into container in readonly mode and an SELinux label

docker run --name fedora -it --volume /home/docker:/mount:ro,Z fedora

• each container has its own network: need to bind container ports to host ports

docker run --name nginx -p 8080:80 -v /home/html:/usr/share/nginx/html:ro,Z nginx

docker start -d <container>
docker stop <container>
docker rm <container>
docker ps
docker ps -a
docker port <container>

Networking

Network types:

• Bridge (default) : on separate 172. network
• host : no network isolation
• none : completely isolated
• ipvlan
• macvlan

Containers can communicate with a bridge network by IP, or better via built-in DNS (via container name) as long as the bridge network is named.

Show network drivers:

docker network ls

Show gateway and IPs that the containers have:

docker network inspect bridge

Create NewNetwork:

docker network create --driver bridge --subnet 192.168.7.0/24 NewNetwork

Storage

on host, in /var/lib/docker

• containers
• image
• volumes

Data is copy-on-write to the container, and never updates the underlying image, so gets lots when the container is destroyed.

Create a volume (in volumes):

docker volume create NewVol

Volume Mount for MySQL container:

docker run -v NewVol:/var/lib/mysqql mysql

Bind Mount to anywhere on the host filesystem with full path:

docker run -v /data/MySQL:/var/lib/mysqql mysql

But the preferred way is:

--mount type=bind,source=/data/MySQL,target=/var/lib/mysql

Create an image

Dockerfile:

# Some comment
FROM baseimage:tag
LABEL org.opencontainers.image.authors="me@example.com"
LABEL version="1.0"
WORKDIR /data
COPY somefile .
ENV PORT 8088
RUN somecommand
ENTRYPPOINT ["python", "manage.py", "runserver"]
CMD ["echo", "Hello World!]

Build:

docker build --rm -t name:tag .

Entrypoints / Commands

• ENTRYPOINT defines the command that is run as PID 1
• CMD adds options to it the entrypoint. If ENTRYPOINT is not specified, "/bin/sh -c" is assumed
• Anything added after image name in "docker run" command is treated as CMD arguments

Environment Variables

Pass into docker run:

docker run -e DBUSER=dbuser -e DBPORT=5432 <container>

Or using a value so it doesn't appear on process list:

docker run -e DBUSER=dbuser -e DBPASS <container>

Or from a file:

docker run --env-file ./env_vars

Push image

• Create a repository on Docker Hub, someuser/somerepo

When building a local image use the tag "someuser/somerepo", or tag an existing local image "local-image" with the dockerhub tag:

docker tag local-image someuser/somerepo

Login to docker (credential store in /root/.docker/config.json):

docker login

Push the image to dockerhub (tagname defaults to latest):

docker push someuser/somerepo:tagname

Push a new image:

docker tag local-image:tagname somerepo:tagname
docker push somerepo:tagname

Debugging

Look at stdout from PID 1:

docker logs -f <container>

File is stored on host (see inspect for path) until container is removed. Has options -f and -n which act like tail.

Attach terminal to container (CTRL-p CTRL-q to exit) and see stdout:

docker attach <container>

Start an interactive shell into the container:

docker exec -it <container> /bin/bash

Pause/unpause:

docker pause <container>
docker unpause <container>

See top for a container or stats for all containers on a host:

docker top container
docker stats

See container definition/state (JSON):

docker inspect <container>

especially:

• Current state of the container. in the “State” property
• Path to the log history file, in the “LogPath” field
• Values of set environment vars, in the “Config.Env” field
• Mapped ports, in the “NetworkSettings.Ports” field

Show container history:

docker history <container>

Override ENTRYPOINT:

docker run -d -p 80:80 --entrypoint /bin/sh /myrepo/mydjangoapp

Docker Compose

TBC