Creating new Docker images

Overview

Teaching: 15 min
Exercises: 15 min

Questions

How can you create your own Docker image?

Objectives

Create a new Docker image.

The previous lesson showed how to run existing docker images. This lesson will demonstrate how to build your own images.

What is a Dockerfile?

A Dockerfile is a text file containing a set of instructions for reproducably building a Docker image

My first Dockerfile

Create a new directory, enter it, and create a file called Dockerfile:

mkdir my-first-dockerfile
cd my-first-dockerfile
<edit> Dockerfile

FROM centos:7
MAINTAINER spli@dundee.ac.uk

RUN yum install -y -q epel-release
RUN yum install -y -q python-pip
RUN pip install omego

FROM: The name of a base image
MAINTAINER: The email of the developer or owner
RUN: Runs a shell command

Build it. -t let’s you give the image a name

docker build -t my-docker-image .

Sending build context to Docker daemon  2.048kB
Step 1/5 : FROM centos:7
 ---> 196e0ce0c9fb
Step 2/5 : MAINTAINER spli@dundee.ac.uk
 ---> 737e0a466759
Step 3/5 : RUN yum install -y -q epel-release
 ---> Running in 3e57c2d63ecc
warning: /var/cache/yum/x86_64/7/extras/packages/epel-release-7-9.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID f4a80eb5: NOKEY
Public key for epel-release-7-9.noarch.rpm is not installed
Importing GPG key 0xF4A80EB5:
 Userid     : "CentOS-7 Key (CentOS 7 Official Signing Key) <security@centos.org>"
 Fingerprint: 6341 ab27 53d7 8a78 a7c2 7bb1 24c6 a8a7 f4a8 0eb5
 Package    : centos-release-7-4.1708.el7.centos.x86_64 (@CentOS)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
 ---> 45b20e1cc15a
Removing intermediate container 3e57c2d63ecc
Step 4/5 : RUN yum install -y -q python-pip
 ---> Running in 582e51a06c3b
warning: /var/cache/yum/x86_64/7/epel/packages/python2-pip-8.1.2-5.el7.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 352c64e5: NOKEY
Public key for python2-pip-8.1.2-5.el7.noarch.rpm is not installed
Importing GPG key 0x352C64E5:
 Userid     : "Fedora EPEL (7) <epel@fedoraproject.org>"
 Fingerprint: 91e9 7d7c 4a5e 96f1 7f3e 888f 6a2f aea2 352c 64e5
 Package    : epel-release-7-9.noarch (@extras)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
 ---> 0630ca1f87a9
Removing intermediate container 582e51a06c3b
Step 5/5 : RUN pip install omego
 ---> Running in 289f0bf4dad2
Collecting omego
  Downloading omego-0.6.4.tar.gz
Collecting yaclifw>=0.1.1 (from omego)
  Downloading yaclifw-0.1.2.tar.gz
Collecting argparse (from yaclifw>=0.1.1->omego)
  Downloading argparse-1.4.0-py2.py3-none-any.whl
Installing collected packages: argparse, yaclifw, omego
  Running setup.py install for yaclifw: started
    Running setup.py install for yaclifw: finished with status 'done'
  Running setup.py install for omego: started
    Running setup.py install for omego: finished with status 'done'
Successfully installed argparse-1.4.0 omego-0.6.4 yaclifw-0.1.2
You are using pip version 8.1.2, however version 9.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
 ---> 1e9496172007
Removing intermediate container 289f0bf4dad2
Successfully built 1e9496172007
Successfully tagged my-docker-image:latest
spli@ docker-carpentry (gh-pages)$

And run

docker run -it my-docker-image omego version

0.6.4

Now build it again:

docker build -t my-docker-image .

Sending build context to Docker daemon  2.048kB
Step 1/5 : FROM centos:7
 ---> 196e0ce0c9fb
Step 2/5 : MAINTAINER spli@dundee.ac.uk
 ---> Using cache
 ---> 737e0a466759
Step 3/5 : RUN yum install -y -q epel-release
 ---> Using cache
 ---> 45b20e1cc15a
Step 4/5 : RUN yum install -y -q python-pip
 ---> Using cache
 ---> 0630ca1f87a9
Step 5/5 : RUN pip install omego
 ---> Using cache
 ---> 1e9496172007
Successfully built 1e9496172007
Successfully tagged my-docker-image:latest

The build is much quicker, and you may have noticed the ` —> Using cache` lines.

More Dockerfile commands

The full list of Dockerfile commands is in the Docker builder documentation. Some of the most common ones are:

COPY: Copies a file (e.g. a script, configuration file, or archive) into the Docker image
USER: Change the user that a command is run as (useful for dropping privileges)
WORKDIR: Change the current working directory
EXPOSE: Lists ports that should be exposed to the outside world
VOLUMES: Directories that should be managed separately from the container (e.g. persistent data that should be kept after the container exits)

Key Points

A Dockerfile is a set of instructions for building a Docker image

Convert a Dockerfile into an image using docker build

docker build has a built-in cache to greatly speed up builds

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