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Creating customized docker images

Whenever docker container is deleted,
all the softwares that we have installed within the container will also be deleted.

If we can save the container as an image, then we can preserve the softwares.

This creation of customized docker images can be done in two ways.

1) using docker commit command
2) using docker file

Using docker commit

docker run –name c11 -it ubuntu

Update apt repository

apt-get update

apt-get install git

TO check the git

git –version

exit

TO save the container as image (snapshot )

docker commit c11 myubuntu

To see the list of images

docker images ( you can see the image which you have created )

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Now lets run the image which we have created

docker run –name c22 -it myubuntu

git –version ( git is pre installed )

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Using docker file

This is a simple text file, which uses predefinied keywords for creating customized docker images.

Key words used in docker file ( case sensitive )

1) FROM — used to specify the base image from which the docker file has to be created.

2) MAINTAINER — This represents name of the organization or the
author who created this docker file.

3) CMD — This is used to specify the initial command that should be executed when the container starts.

4) ENTRYPOINT – used to specify the default process that should be executed when container starts.
It can also be used for accepting arguments from the CMD instruction.

5) RUN — Used for running linux commands within the container. It is generally helpful for installing the software in the container.

6) USER — used to specify the default user who should login into the container.

7) WORKDIR —
Used to specify default working directory in the container

8) COPY — Copying the files from the host machine to the container.

9) ADD — Used for copying files from host to container, it can also be used for downloading files from remote servers.

10) ENV — used for specifying the environment variables that should be passed to the container.

EXPOSE — Used to specify the internal port of the container
VOLUME — used to specify the default volume that should be attached to the container.
LABEL — used for giving label to the container
STOPSIGNAL — Used to specify the key sequences that have to be passed in order to stop the container.

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Create a dockerfile by taking nginx as the base image
and specify the maintainer as logiclabs. Construct an image from the dockerfile.

Creating customized docker images by using docker file.

$ sudo su –

vim dockerfile

FROM nginx
MAINTAINER logiclabs

:wq

TO build an image from the dockerfile

docker build -t mynginx .

( t stands for tag,
. stands for current working dir
mynginx is the new image name )

TO see the image

docker images

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When ever i start my container, i want a program to get executed.

vim dockerfile

FROM centos
MAINTAINER logiclabs
CMD [“date”]

:wq

TO build an image from the dockerfile

docker build -t mycentos .

TO see the image

docker images

Running conainer from the image

docker run -it mycentos

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In one docker file, we can have one CMD instruction.
If we give two CMD instruction, it executes the latest one
Lets try

vim dockerfile

FROM centos
MAINTAINER logiclabs
CMD [“date”]
CMD [“ls”, “-la”]

:wq

docker build -t mycentos .

docker run -it mycentos

( Observation, we get ls -la output )

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In ubuntu container, I want to install git in it.

Lets remove the docker file

rm dockerfile

vim dockerfile

FROM ubuntu
MAINTAINER logiclabs
RUN apt-get update
RUN apt-get install -y git

:wq

Note: CMD — will run when container starts.
RUN — will executed when image is created.

docker build -t myubuntu .

Lets see the images list and space consumed by our image

docker images

docker run -it myubuntu

git –version

exit

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Cache busting

Whenever an image is build from a dockerfile, docker reads its memory and checks which instructions were already executed.
These steps will not be reexecuted.
It will execute only the latest instructions. This is a time saving mechanism provided by docker.

But, the disadvantage is, we can end up installing software packages from a repository which is updated long time back.

Ex:

cd docker

vim dockerfile

Lets just add one more instruction

FROM ubuntu
MAINTAINER logiclabs

RUN apt-get update
RUN apt-get install -y git
RUN apt-get install -y tree

:wq

Lets build an image

docker build -t myubuntu .

( Observe the output, Step 2, 3, 4 is using cache. Only step 5 is executed freshly )

Advantage: time saving mechanism

Disadvantage : Lets say, you are running after 4 months, We are installing tree from apt which is updated long time back. )

TO avoid this disadvanatge we use cache busting

Note: cache busting is implemented using && symbol.
Which ever statement in the docker file has && will be re-executed.

vim dockerfile

FROM ubuntu
MAINTAINER logiclabs

RUN apt-get update && apt-get install -y git tree

:wq

Lets build an image

docker build -t myubuntu .

( Observe the output, step 3 – It is not using cache )

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