Docker 2

Docker recipes: build your own images

OS commands in image building

Depending on the underlying OS, there are different ways to build images.

Know your base system and their packages. Popular ones:

Debian
CentOS
Alpine
Conda. Anaconda, Conda-forge, Bioconda, etc.

Update and upgrade packages

In Ubuntu:

apt-get update && apt-get upgrade -y

In CentOS:

yum check-update && yum update -y

Search and install packages

In Ubuntu:

apt search libxml2
apt install -y libxml2-dev

In CentOS:

yum search libxml2
yum install -y libxml2-devel.x86_64

Note the -y option that we set for updating and for installing.<br> It is an important option in the context of Docker: it means that you answer yes to all questions regarding installation.

Building recipes

All commands should be saved in a text file, named by default Dockerfile.

Basic instructions

Each row in the recipe corresponds to a layer of the final image.

FROM: parent image. Typically, an operating system. The base layer.

FROM ubuntu:18.04

RUN: the command to execute inside the image filesystem.

Think about it this way: every RUN line is essentially what you would run to install programs on a freshly installed Ubuntu OS.

A basic recipe:

FROM ubuntu:18.04

RUN apt update && apt -y upgrade
RUN apt install -y wget

docker build

Implicitely looks for a Dockerfile file in the current directory:

docker build .

Same as:

docker build --file Dockerfile .

Syntax: –file / -f

. stands for the context (in this case, current directory) of the build process. This makes sense if copying files from filesystem, for instance. IMPORTANT: Avoid contexts (directories) overpopulated with files (even if not actually used in the recipe).

You can define a specific name for the image during the build process.

Syntax: -t imagename:tag. If not defined `:tag` default is latest.

docker build -t mytestimage .
# Same as:
docker build -t mytestimage:latest .

IMPORTANT: Avoid contexts (directories) over-populated with files (even if not actually used in the recipe).

In order to avoid that some directories or files are inspected or included (e.g, with COPY command in Dockerfile), you can use .dockerignore file to specify which paths should be avoided. More information at: https://codefresh.io/docker-tutorial/not-ignore-dockerignore-2/

The last line of installation should be Successfully built …: then you are good to go.

Check with docker images that you see the newly built image in the list…

Then let’s check the ID of the image and run it!

docker images

docker run f9f41698e2f8
docker run mytestimage

More instructions

WORKDIR: all subsequent actions will be executed in that working directory

WORKDIR ~

ADD, COPY: add files to the image filesystem

Difference between ADD and COPY explained here and here

COPY: lets you copy a local file or directory from your host (the machine from which you are building the image)

ADD: same, but ADD works also for URLs, and for .tar archives that will be automatically extracted upon being copied.

If we have a file, let’s say `example.jpg`, we can copy it.

# COPY source destination
COPY example.jpg .

A more sophisticated case:

FROM ubuntu:18.04

RUN apt update && apt -y upgrade
RUN apt install -y wget

RUN mkdir -p /data

WORKDIR /data

COPY example.jpg .

ENV, ARG: run and build environment variables

Difference between ARG and ENV explained here.

ARG values: available only while the image is built.
ENV values: available during the image build process but also for the future running containers. * It can be checked in a resulting running container by running env.

In the case below UbuntuVersion argument is provided with a default value.

ARG UbuntuVersion=18.04

FROM ubuntu:${UbuntuVersion}

Override the value for UbuntuVersion as you build the image with –build-arg:

docker build --build-arg UbuntuVersion=20.04 .

We try a simple example with ENV

FROM ubuntu:18.04

ENV PLANET="Earth"

RUN echo ${PLANET}

Enter in the container interactively and check variable ${PLANET}

You can pass variable through commandline as well (with --env/-e) during running process:

docker run -ti --env PLANET=Mars test

Try to replace the examples above of ENV with ARG and see what happens.

CMD, ENTRYPOINT: command to execute when generated container starts

The ENTRYPOINT specifies a command that will always be executed when the container starts. The CMD specifies arguments that will be fed to the ENTRYPOINT

In the example below, when the container is run without an argument, it will execute echo “hello world”. If it is run with the argument hello moon it will execute echo “hello moon”

FROM ubuntu:18.04
ENTRYPOINT ["/bin/echo"]
CMD ["hello world"]

A more complex recipe (save it in a text file named Dockerfile):

FROM ubuntu:18.04

MAINTAINER Toni Hermoso Pulido <toni.hermoso@crg.eu>

WORKDIR ~

RUN apt-get update && apt-get -y upgrade
RUN apt-get install -y wget

ENTRYPOINT ["/usr/bin/wget"]
CMD ["https://cdn.wp.nginx.com/wp-content/uploads/2016/07/docker-swarm-hero2.png"]

docker run f9f41698e2f8 https://cdn-images-1.medium.com/max/1600/1*_NQN6_YnxS29m8vFzWYlEg.png

docker tag

To tag a local image with ID “e23aaea5dff1” into the “ubuntu_wget” image name repository with version “1.0”:

docker tag e23aaea5dff1 ubuntu_wget:1.0

Build exercise

Random numbers
Copy the following short bash script in a file called random_numbers.bash.

#!/usr/bin/bash
seq 1 1000 | shuf | head -$1

This script outputs random intergers from 1 to 1000: the number of integers selected is given as the first argument.

Write a recipe for an image:
- Based on centos:7
- That will execute this script (with bash) when it is run, giving it 2 as a default argument (i.e. outputs 2 random integers): the default can be changed as the image is run.
- Build the image.
- Start a container with the default argument, then try it with another argument.