DevOps Classroom Series – 01/Nov/2020

Consideration for building Docker image for any application

• Right base image
• Steps for installing and configuring your application
• Expressing details about the ports on which your application works
• Commands to start your application.

Docker images in depth

• Docker image is collection of layers.
• Theroitically each layer is result of the change (RUN, COPY, ADD) will lead to creation of image layers
• Lets take the openjdk image docker image pull openjdk:8 and execute docker image inspect openjdk:8
• Lets try to create our new docker image with openjdk:8 as base image

FROM openjdk:8

• Now lets inspect myjdk:8 and look into layers
• myjdk:8 is exactly the collection of 7 image layers which openjdk:8 has
• Lets add some stuff to this image

FROM openjdk:8
RUN wget https://referenceappkhaja.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar

• now lets inspect myjdk:8.1 for image layers
• myjdk 8.1 add approx 40 mb of file to existing openjdk:8 in the new layer added
• If the image layer which is used by new image which you are pull already exists docker image will not download it again
• Docker images can be built from scratch Refer Here
• docker image is collection of image layers
• Then what will container have one extra read write layer for the changes

Dockerfile IDE

• For developing Dockerfile any text editor can be used.
• In this series i will be using visual studio code + Docker extension from Microsoft added

Dockerfile instructions

• Refer Here for the Dockerfile official docs

• FROM: Using from instruction we select baseimage, always choose a base image with some tag not latest Refer Here

• ADD and COPY: Are used to copy the contents into the docker image. ADD instruction can download the content from internet also (URI/URL).

  • Refer Here for add instruction and Refer Here for copy instruction

• EXPOSE: instruction lets Docker know that when the image is execute, the port and protocol defined will be exposed at runtime Refer Here

• Refer Here for the sample used

• ENTRYPOINT and CMD:

Docker container creation

• when ever you want to create a container execute run command

docker container run <image>:<tag>

• The entrypoint/cmd gets executed and as long as that command is executing container will be alive
• Refer Here for the sample docker image
• Now lets run this image and check for the containers
• Lets look at all the container status
• docker container has executed echo hello-world which completed its activity & now docker engine since the command execution is completed stopped the container.
• Now lets run the spc:trail1 with different cmd options
• Whatever command which is passed after image:tag in docker container run will replace CMD instruction
• Refer Here for the changes done
• Build the docker image
• In Docker entrypoint and CMD instruction can be written in two formats
  • Exec form: Refer Here
  • Shell form: Refer Here
• Docker container running modes:
  • attached: This is default mode
  • detached/background: in this mode your container will just show the container id and run in background

  docker container run -d <image>:<tag>
  

  • interactive: In this mode we login into the terminal of container

  docker container run -it <image>:<tag> <terminalpath>
  

  
• Docker container is a process isolation.
  • It gets cpu and memory
  • It gets storage (mount of image layers+read write layer)
  • It gets network interface
• Network port mapping b/w docker host and containers
• Exercise: Try building a docker image for
  • nop commerce: Refer Here take ubuntu:18 as a base image
  • openmrs standalone: Refer Here