Docker Image Creation and Docker Image Layers

Dockerfile Introduction

• In this section we would cover Dockerfiles and instructions.
• Dockerfile is a simple plain text file that contains set of user defined instructions which will be call during building the image.
• Click here for Dockerfile reference

Syntax

• Generic Syntax is INSTRUCTION arguments for eg FROM alpine or RUN echo 'Hello'
• Dockerfile should start with a FROM instruction which specifies the base image.

Docker File Instructions

• FROM:

  • Syntax is FROM <image>[:tag] [As <name>].
  • note: any thing in [] above are optional
  • This instruction sets the base image.
  • Multiple FROM statements are also allowed to support multistage builds
  • Refer here for more info

• LABEL :

  • Adds metadata to the image
  • Refer Here for more info.

• RUN :

  • This command takes arguments in two forms
    • Shell form: RUN <command>
      • In this command runs in a shell
    • exec form: RUN ["executable", "param1", "param2" ]
      • In this way comand execution doesn’t require a defaault shell
  • Each RUN instruction will execute the commands in the new layer on top of current image
  • Refer Here for more info.

• COPY

  • copies file from soure to destination.
  • Syntax: COPY <src> <dest>
  • src in above syntax is only files and directories . URL’s are not supported
  • Refer Here for more info

• ADD

  • copies file from soure to destination.
  • Syntax: ADD <src> <dest>
  • src in above syntax is only files and directories . URL’s are supported as source.
  • Refer Here for more info

• ENTRYPOINT

  • Command which gets executed when the docker container is created from image.
  • CMD acts as argument to the ENTRYPOINT
  • Refer Here for more info

• CMD

  • Refer Here for more info
  • TO understand CMD and ENTRYPOINT interatction refer here

• EXPOSE

  • Refer Here

Building an Image

• Lets build a spring-pet-clinic app image

• Manually to execute spring pet clinic

  • Clone from Github using the following HTTPS url. git clone https://github.com/spring-projects/spring-petclinic.git

  • Ensure Java and maven are installed.

  • Execute the following command to build the image mvn package

  • Copy spring-petclinic*.jar file from target folder to any folder in your linux machine.

  • Execute the following command to run the container “` java -jar spring-petclinic*.jar“

• From the above understanding lets write the Dockerfile

  • Create a new directory called as spring-petclinic
  • Create a file called as Dockerfile
  • Copy the spring-petclinic.jar to samefolder
  • Insert the following contents

  FROM openjdk:8
  LABEL author="KHAJA"
  LABEL version="1.0"
  COPY ./spring-petclinic.jar /spring-petclinic.jar
  EXPOSE 8080
  ENTRYPOINT ["java", "-jar"] 
  CMD ["/spring-petclinic.jar"]
  

  • Execute the docker image build command docker image build -t springpetclinic:1.0 .
  • Try to create container with following command

  docker container run -d -p 8080:8080 springpetclinic:1.0
  

• Lets examine the layers

  1. Execute the following command docker image inspect openjdk:8
  2. Navigate to Layers and have a look at highlighted layers
  3. Total we have seven layers
  4. Now execute the following command docker image inspect springpetclinic:1.0 and Navigate to layers section

• The result of the observations above are

  • We had one extra copy instruction which had led to creation of new layer for the image springpetclinic:1.0 (which is highlighted in yellow at the bottom)
  • Every instruction (RUN, ADD, COPY etc ) leads to the creation of new image layers

• All of these layers are combined in the image & as well as container.

• These layers are readonly layers