DevOps Classroomnotes 02/Sep/2023

Docker contd

Layers in Docker Image

• Docker image is collection of read only image layers and meta-data

Experiments

• Pull the alpine:3.17 image and inspect and focus on layers

docker image pull alpine:3.17
docker image inspect alpine:3.17

* Pull the jenkins/jenkins image and inspect and focus on layers

docker image pull jenkins/jenkins
docker image inspect jenkins/jenkins

* Use this Dockerfile and build the image with name exp:1.0

FROM alpine:3.17

* Now both alpine and exp:1.0 have same layers used
* Now consider the below dockerfile with name exp:1.1

FROM alpine:3.17
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/gameoflife.war /gameoflife.war

* Now as observed a new layer was created in exp:1.1

* Now build the exp:1.2 with the following Dockerfile

FROM alpine:3.17
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/gameoflife.war /gameoflife.war
RUN mkdir test
RUN cp /gameoflife.war /test/gameoflife.war

* Writing too many RUN instructions is not a good idea, so combine them. Use \ to increase Readability

FROM alpine:3.17
ADD https://referenceapplicationskhaja.s3.us-west-2.amazonaws.com/gameoflife.war /gameoflife.war
RUN mkdir test && \
    cp /gameoflife.war \
    /test/gameoflife.war \
    && java -version > version.txt

• Docker container when created will union all the image layers and one writable layer to create a logical disk. This is done by Storage Drivers. The most popular one is overlay2.
  
• Refer Here for the article explaining layers and drivers
• When we delete the container writable layer is deleted. To solve this we will learn docker volumes.

ENTRYPOINT and CMD in docker files

• Docker container when starting will use the following to formulate the start up command

ENTRYPOINT CMD # Both Entrypoint and CMD exists in Docker
ENTRYPOINT
CMD

• Whatever we have written in CMD can be overriten by passing arguments after image name during run/create
• The Dockerfile used is

FROM alpine:3.17
CMD ["sleep", "1d"]

• The Dockerfile is

FROM alpine:3.17
ENTRYPOINT ["sleep"]
CMD ["1d"]

* To chage entrypoint

* Difference between fork vs exec linux
* Difference between shell form and exec form in Dockerfile
* references
* Refer Here
* Refer Here

• Problem: Docker container deletes the writable layer when deleted i.e. all the changes done by the contianer is lost

Docker volumes

• Refer Here for blog article
• We can be explicit and create volumes when our container is created automatically by adding VOLUME instruction in dockerfile
  
• Now create the container some-mysql and look at volumes

docker run --name some-mysql -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mysql

* Now create a volume and mount it it mysql with same specific-mysql