Docker-Dockerfile

  Docker can build images automatically by reading the instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.

Some of the Keyword's definition

FROM

is define th base image on which we are building  eg FROM ubuntu

ADD

is used to add the files to the container being built, ADD <source> <destination in the container>

RUN

is used to add layers to the base image, by installing components.Each RUN statement add a new layer to the docker image

CMD

is used to run the command on start of the container.These commands run when there is no argument specified while running the container.

ENTRYPOINT

is used to strictly run the commands the moment the container intializes. The difference between CMD and ENTRYPOINT is, ENTRYPOINT runs  irrespective of the fact that whether the argument is specified or not.

ENV

is used to define the environment in container.

Docker build

Description

Build an image from a Dockerfile

sudo docker build -t nonrootimage . # create custom image (nonrootimage)

Examples.

Create an image which has base image ubuntu and apache2 is to be installed on it and create an index.html file in current directory,  all the files from the current directory is to be copied to /var/www/html folder. Once the container is started it should run the apache service and also create one environment variable called "name" and it should have value "DEVOPS

FROM ubuntu

ARG DEBIAN_FRONTEND=noninteractive

RUN apt-get update

RUN apt-get -y install apache2

ADD . /var/www/html

ENTRYPOINT apachectl -D FOREGROUND

ENV name DEVOPS 

Run below command to build the image

docker build . -t  img1   #Created the image from above Dockerfile

Example 2

Create a Docker file which uses a base image of CentOS 7 and create a user john and change to non-root privilege

# Base image is CentOS 7

FROM centos:7

# Add a new user "john" with user id 8877

RUN useradd -u 8877 john

# Change to non-root privilege

USER john

Example 3

#Eample of COPY and ADD

FROM centos:7.4.1708

RUN mkdir /mydata

COPY myfiles /mydata/myfiles  

ADD myfile2 /mydata/myfile2  

ADD https://mirrors.estointernet.in/apache/maven/maven-3/3.6.3/binaries/apache-maven-3.6.3-bin.tar.gz /mydata 

ADD apache-maven-3.6.3-src.tar.gz /mydata/maven

# CMD and ENTRYPOINT

FROM ubuntu

CMD echo "Hello World"

docker build . -t  img1   #Created the image from above Dockerfile

docker run -it img1 # it will return Hello World

docker run -it img1 echo "Hello India" # it will overwrite the CMD and Print Hello India

FROM ubuntu

ENTRYPOINT ["echo","Hello World"]

docker build . -t  img1   #Created the image from above Dockerfile

docker run -it img1 # it will return Hello World

docker run -it img1 echo "Hello India" # it will not overwrite the ENTRYPOINT and Print Hello World echo Hello India

FROM ubuntu

ENTRYPOINT ["echo"]

CMD ["Hello World"]

docker build . -t  img1   #Created the image from above Dockerfile

docker run -it img1 # it will return Hello World

Note:- if the file name is not Dockerfile

docker build . -f abc -t img8 # abc is the file name which represents the dockerfile contents