DevOps Classroom Series – 26/Mar/2021

ENV & ARG Directive/instruction

  • The ENV directive is used to set environmental variables.
  • These environmental variables are used by applications to get information
  • Syntax
ENV <key> <value>
  • ARG instruction is used to define the variables that the user can pass while building the image
  • Syntax
ARG <variablename>=<defaultvalue>
  • ENV variables will be available in the containers also
  • Refer Here for the changeset containing the dockerfile
  • Now while building the images using docker file use the following commands and explore the differences
docker image build -t envargdemo:1.0 --build-arg downloadlocation='https://referenceappkhaja.s3-us-west-2.amazonaws.com/spring-petclinic-2.2.0.BUILD-SNAPSHOT.jar' .
# login into the container to see the file downloaded and the environmental variables
docker container run -it envargdemo:1.0 /bin/bash
ls
echo $dummy
echo $applicationpath
exit
# Create one more container
docker container run -e dummy='test' -it envargdemo:1.0 /bin/bash
ls
echo $dummy
echo $applicationpath
exit

WORKDIR instruction

  • In the below image we ran two different containers where the working directory of the containers are different Preview
  • By default if we donot specify the default working directory is /
  • Where as in Dockerfile it is possible to change the working directory
  • Refer Here for the changeset Preview

ADD and COPY instruction

  • During the docker image build process we might require to copy files from our local system into docker image filesystem. This can be acheived by using COPY/ADD instruction
COPY <source> <destination>
ADD <source> <destination>
  • In some cases while building docker image you might require to download the files from some urls and copy to the docker file system, then we can use ADD instruction
ADD <source> <destination>
  • Refer Here and try to build the docker image and check the behavior of COPY and workdir

USER instruction

  • Look at the example below Preview
  • So there is some way to set the default user other than root user in DockerImage. This can be achieved by using USER instruction
  • Will be continued in the next class

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