AGENDA:
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- Dockerfile instructions
- CMD vs ENTRYPOINT
- Registry in dockerhub
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ex1: create an image named ubuntujava with tag as v1
take ubuntu as the base image, update apt package & install java (or default-jre)
Whenever i start my container, echo comamnd to get executed

FROM ubuntu
RUN apt update -y
RUN apt install default-jre -y
CMD ["echo","i have created image / container with java installed"]

#observation: build the image & run container, once container gets started observe the CMD instruction gets executed (echo command)

Note:
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as best practice in CMD & ENTRYPOINT instructions use them inside square brackets with comma seperation,
i.e exec form ==> ["<commands>"]
ex: ["echo","hello world"]

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Instructions in Dockerfile (continued....)
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8) COPY --> Copying the files from the dockerhost machine to the container.
syntax: COPY <files/Directory_name_in_dockerhost> <path_inside_container>

9) ADD --> Used for copying files from host to container, it can also be used for downloading files from remote servers.
synatx: ADD <files/Directory_name_in_dockerhost> <path_inside_container>

ADD https://dlcdn.apache.org/tomcat/tomcat-9/v9.0.82/bin/apache-tomcat-9.0.82.tar.gz /opt

What is difference between COPY & ADD instruction in Dockerfile?
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Both COPY & ADD instructions are used to copy files/Directories from DockerHost to Container.
ADD can also be used to download file (like wget command in linux) & also ADD can untar/unzip file inside containers,

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

ENV <Variable_name> <variable_value>

ENV sportsman viratkohli

inside container ==> echo $sportsman

11) EXPOSE -- Used to specify the port on which we want to run our container

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What is difference between CMD & ENTRYPOINT instruction in Dockerfile:
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CMD ==> we will define the commands that needs to be executed when a container starts.
ENTRYPOINT ==> we will define the commands that needs to be executed when a container starts.

*****important_interview_question*************

Difference between CMD & ENTRYPOINT ?
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CMD sets the default commands that needs to be executed when a container starts.
CMD command/instruction can be easily overridden while creating a container with different command.
whereas ENTRYPOINT command/instruction can't be overridden while creating a container.

Most of Docker containers by default have ENTRYPOINT ==> which is ==> /bin/sh -c

ENTRYPOINT generally used if we want to container as an executable ( like only purpose of running container is to run a script only)

Can we have both CMD & ENTRYPOINT in docker file?
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Yes we can have both in a Dockerfile. but CMD instructions will be passed as an arguments for ENTRYPOINT.

ex: usecase1 of using both CMD & ENTRYPOINT?
FROM ubuntu
CMD ls
ENTRYPOINT ["echo", "Helloworld"]

build above Dockerfile, run container ==> container Will be executed as below ===> Helloworld ls

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Scenario 1:
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Create a dockerfile by taking
- ubuntu as the base image
- create a user as bharath & make him as default user after logging in container
- default working directory as opt
Construct an image from the dockerfile.

vi Dockerfile

FROM ubuntu
RUN useradd Bharath
CMD ["/bin/bash"]
ENTRYPOINT ["echo", "Helloworld"]
USER bharath
WORKDIR opt
RUN apt update -y && apt install default-jre -y && apt install maven -y && apt install git -y
COPY ./samplefile /opt
ADD https://dlcdn.apache.org/tomcat/tomcat-11/v11.0.0-M26/bin/apache-tomcat-11.0.0-M26.zip /opt
ENV king kohli

 

or 

FROM ubuntu
RUN useradd Bharath
CMD ["/bin/bash"]
ENTRYPOINT ["echo", "Helloworld"]
USER bharath
WORKDIR opt
COPY ./samplefile /opt
ADD https://dlcdn.apache.org/tomcat/tomcat-11/v11.0.0-M26/bin/apache-tomcat-11.0.0-M26.zip /opt
ENV king kohli

docker build -t <image_name> .

create conatiner & verify

Scenario 2:
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Create a dockerfile by taking
- alpine as the base image
- copy samplefile to docker container to /opt directory
Construct an image from the dockerfile.
--> create a file called samplefile in dockerHost

vi Dockerfile
FROM alpine
COPY ./samplefile /opt

docker build -t <image_name> .

create container & verify

Scenario 3:
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Create a dockerfile by taking
- busybox as the base image
- download maven installation file to docker conatiner /opt directory
Construct an image from the dockerfile.

FROM centos
ADD https://dlcdn.apache.org/maven/maven-3/3.8.6/binaries/apache-maven-3.8.6-bin.tar.gz /opt

Note:
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Alpine & busybox are ligh weight (smaller sized) docker images which will have all linux utilities

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How long will a docker container run?
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Every docker image come with default process.
As long as default process is running, the container will be running condition. The moment, the default process execution is completed, the container will get itself moved to exited / stopped state.

(default process ==> whatever mentioned in CMD instruction in Dockerfile)
in already running containers, we can check default process using docker ps command & observe under COMMAND sections

Practicals & observations on understanding default process in containers:
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scenario 1:
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Create Dockerfile with below mentioned instructions

FROM ubuntu
CMD ["date"]

build this Dockerfile & create a conatiner from it & observe that conatiner has moved in to exited state.

Reason? ==> container has exited, because it has completed running its default process (whatever mentioned in CMD instruction i.e date command )

For all linux based containers( ubuntu,centos,amazonlinux.....) , the default process is shell process for that Dockerfile will look like

FROM ubuntu
CMD ["/bin/bash"]

/bin/bash or bash -- is nothing but the terminal.
Hence we are able to enter -it mode in ubuntu/centos or any OS based containers.

 

alltogether

FROM ubuntu
RUN useradd bharath
USER bharath
WORKDIR opt
COPY ./samplefile /opt
ADD https://dlcdn.apache.org/tomcat/tomcat-11/v11.0.0-M26/bin/apache-tomcat-11.0.0-M26.zip /opt
ENV king kohli

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Kubernates