DevOps Classroom notes 28/Jun/2025

Dockerfile

Asp.dotnet core application containerization

• Refer Here for the repo
• The Dockerfile only building the code is

From mcr.microsoft.com/dotnet/sdk:9.0 As build
Add . /nop
WORKDIR /nop
RUN dotnet build -c Release src/Presentation/Nop.Web/Nop.Web.csproj

• Refer Here for the full version
• Exercise: Ensure we have a user created in Dockerfile as of now we are running application as root user.

SHELL FORM and EXEC FORM

• shell form

RUN apt update
CMD java -jar spc.jar

• EXEC form

CMD ["java", "-jar", "spc.jar"]

• SHell form any command is executed with /bin/sh -c <command> and this does not react well with signals sigint
• Exec form directly executes the command which makes application PID-1 in a container and handles signals. You can expand variables in EXEC form
• Use Shell form for RUN and EXEC form for ENTRYPOINT and CMD

Other Dockerfile instructions to consider

• ENTRYPOINT/CMD
• ARG
• ENV

While building docker image i want to pass variables

• Consider the following docker file

FROM openjdk:17
ADD https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar /spring-petclinic-3.3.0-SNAPSHOT.jar
EXPOSE 8080
CMD java -jar /spring-petclinic-3.3.0-SNAPSHOT.jar

• Refer Here for ARG instruction

FROM openjdk:17
ARG JAR_URL='https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar'
ADD ${JAR_URL} /spring-petclinic-3.3.0-SNAPSHOT.jar
EXPOSE 8080
CMD ["java", "-jar", "/spring-petclinic-3.3.0-SNAPSHOT.jar"]

• build command

docker image build --build-arg JAR_URL='https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar' -t spc:1 .

• ARGs are accesible only while building image not while running container.

I Want to set values which i will be using in a container

• Lets consider the following

FROM openjdk:17
ARG JAR_URL='https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar'
ADD ${JAR_URL} /spring-petclinic-3.3.0-SNAPSHOT.jar
EXPOSE 8080
CMD ["java", "-jar", "/spring-petclinic-3.3.0-SNAPSHOT.jar"]

• ENV

FROM openjdk:17
ARG JAR_URL='https://referenceappslt.s3.ap-south-1.amazonaws.com/spring-petclinic-3.3.0-SNAPSHOT.jar'
ADD ${JAR_URL} /spring-petclinic-3.3.0-SNAPSHOT.jar
ENV TEST='DEFAULT'
EXPOSE 8080
CMD ["java", "-jar", "/spring-petclinic-3.3.0-SNAPSHOT.jar"]

ENTRYPOINT and CMD

• CMD can be overriten while running the docker by passing any thing after image name during run

docker run ..... <image> [------]

• ENTRYPOINT can be change via –entrypoint during run
• When you run a docker container it will run ENTRYPOINT + CMD as PID 1
• Case 1: This will execute echo hello when container starts

FROM alpine
ENTRYPOINT [ "echo" ]
CMD ["hello"]

• Case 2: Only entrypoint

FROM alpine
ENTRYPOINT [ "echo", "hello" ]

• Case 3: Only CMD

FROM alpine
CMD [ "echo", "hello" ]

• Case 4: NO CMD and NO Entrypoint the container will try to run base images Start command

Optimizations in Dockerfile

• Storage
• Security