Docker - For Beginner Part 4

It is a text file that consists of a set of instructions/sets of tasks.
It is used for automation which saves time.
You can't do changes in images but you can do modifications in the docker file and create a new image.

Docker Components :

1. FROM :
   It is the topmost field of the file.
   It states the base image of the container.
   e.g. FROM ubuntu

2. RUN :
   It executes the commands.
   As we add the commands, it creates the layer of the image.
   e.g. RUN echo "Hello Pranav" > /tmp/test.txt

3. COPY :
   It is used for copying the files from the local machine.
   You can't use remote files for copying.
   e.g. COPY <source> <destination>

4. ADD :
   It is similar to COPY, but it provides a feature to download files from a remote location.
   It is also used for extracting the archive files.
   e.g. ADD <source> <destination>

5. EXPOSE :
   It exposes the port to the docker.
   It decides which port should be open for that container.
   e.g. EXPOSE 8080

6. WORKDIR :
   The WORKDIR command is used to define the working directory of a Docker container at any given time.
   e.g WORKDIR /var

7. CMD :
   It sets the default parameters which are overridden by the Docker Command Line Interface (CLI) when a container is running.
   e.g. CMD yum install httpd -y

8. ENTRYPOINT
   Default parameters that cannot be overridden when Docker Containers run with CLI parameters.
   e.g. ENTRYPOINT yum update -y

9. VOLUME
   It is a directory somewhere in your Docker storage directory and can be mounted to one or many containers.
   e.g. VOLUME /var/log/app

10. ENV
    For declaring the environmental variable we use "ENV".
    e.g. ENV myname=Pranav

11. ARG
    It is used for providing arguments to the commands.
    You can use this as a Parameter and Default Value for it.
    e.g. ARG Default_Passing_Marks

12. USER
    The USER command sets the startup user ID or user name in the new image.
    e.g. USER <UserId>
    e.g. USER <UserName>

13. MAINTAINER
    This will notify the owner of the file i.e Who has written the Docker File.
    MAINTAINER <email-id>

Docker File Example

#This is a sample Image 
FROM ubuntu 
MAINTAINER mytestmail@gmail.com 

RUN apt-get update 
RUN apt-get install –y nginx 
CMD [“echo”,”Image created”]

#This is a sample Image 
FROM ubuntu 
MAINTAINER mytestmail@gmail.com 

ENV Home_Dir=/root/
COPY /var/log/test.log /tmp
WORKDIR /var
ADD /var/log/all_log.tar.gz /tmp

#This is a sample Image 
FROM ubuntu 
MAINTAINER mytestmail@gmail.com 

VOLUME /var/log 
EXPOSE 8080
USER docker_user
ENTRYPOINT yum update -y

Note:
Special thanks to Pranoti Parkale & Ritik Khode for their help.