Dockerfile

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Dockerfile Instructions

ENTRYPOINT vs CMD

ENTRYPOINT

It sets the main command to be executed when running the container.

ENTRYPOINT ["command", "param1", "param2"]

You cannot override ENTRYPOINT without --entrypoint flag.

CMD

It provides default arguments for the ENTRYPOINT or the command to run if no ENTRYPOINT is specified.

CMD ["command", "param1", "param2"]

You can override CMD by providing command line arguments when running the container e.g.:

docker run my_image othercommand otherparam

ENTRYPOINT + CMD

Putting them together allows you to set a fixed command with default parameters that can be overridden.

ENTRYPOINT ["python"]
CMD ["app.py"]

docker run my_image          # will run `python app.py`
docker run my_image other.py # will run `python other.py`

ARG vs ENV

ARG

Variables for build-time (only available during build time).

ARG MY_NAME=Alex

ENV

Environment variables for runtime (available when container is running).

These environment variables are available during build time and runtime.

ENV MY_NAME=Alex

From

It specifies the base image for the Dockerfile.

FROM <IMAGE_NAME>:<TAG>

Create Dockerfile

touch Dockerfile

FROM jenkins/jenkins:2.506-slim-jdk17    # use official Jenkins image as base

USER root

RUN apt-get update && apt-get install -y \    # install dependencies
    git \
    maven

USER jenkins

RUN jenkins-plugin-cli --plugins \
    "warnings-ng" \
    "sonar"

Build Image from Dockerfile

docker build -t <NAME>:<TAG> <PATH_TO_DOCKERFILE>