DevOps Classroom Series – 24/Aug/2021

Dockerfile Instructions and Image Creations

• To create a docker container we need an image.

• Applications will have different versions, To accomodate this in docker container images, Docker images have tags.

• The syntactic name of docker image <image-name>:<tag-name>.

• If we don’t pass the tag-name the default tag latest will be applied.

• To create any Docker image, we need to know the manual steps for configuring and Running the application.

• To make the process of creating docker images simple, Docker has `Dockerfile“ which is text file and contains instructions on how to build a container image.

• This is declarative way of building images

• Refer Here for the Dockerfile instructions and reference

• Lets start with simple instructions

  • FROM:
    • Using this we define what is the base image for building our custom application image

    FROM <base-image>:<tag>
    

    • It is recommended not to use the latest tag prefer specific tags over here
    • If you really want to start to build a custom image with out any specific base image FROM scratch
    • Refer Here
  • LABEL: Adds metadata to the image Refer Here
  • ADD: This instruction can be used to copy the files from http or docker host into docker image

  ADD <source> <destination>
  

  • COPY: This instruction can be used to copy the files from docker host into docker image

  COPY <source> <destination>
  

  • RUN
  • EXPOSE: Expose instruction infor Docker that the container is listenting to the specific netowrk ports

  EXPOSE <port> [<port>/<protocol>]
  

  • CMD: This instruction executes the command when the container is created Refer Here

• The Dockerfile for springpetclinic which we have created is

FROM openjdk:11
LABEL author="khaja ibrahim"
LABEL organization="quality thought"
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar /spring-petclinic.jar
EXPOSE 8080
CMD [ "java", "-jar", "/spring-petclinic.jar" ]

• Now lets try to build a docker image

docker image build -t <image-name>:<tag> <path to folder where Dockerfile>

docker image build -t <image-name>:<tag> -f <path to Dockerfile>

• We can alos use the slim versions from base image if available

FROM openjdk:11-slim
LABEL author="khaja ibrahim"
LABEL organization="quality thought"
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/spring-petclinic-2.4.2.jar /spring-petclinic.jar
EXPOSE 8080
CMD [ "java", "-jar", "/spring-petclinic.jar" ]

• Exercise: Try create an docker container image for gameoflife
  • Package file: https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/gameoflife.war
  • This file will run on tomcat 9 version with java version 8 and this war file has to be copied into the webapps folder of tomcat
  • This application will use tomcat’s port which is 8080

FROM tomcat:jdk8
LABEL author="khaja"
EXPOSE 8080
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/gameoflife.war /usr/local/tomcat/webapps/gameoflife.war