Dockerfile

A Dockerfile is where you’ll estabilish the definitions used to build a container image. It uses three main keywords/commands:

FROM: where you’ll inform the base image used to build your own image
COPY: where you’ll add resources (files) to your image
CMD: where you’ll inform how to start the application

Sometimes people ask when to use ADD and when to use COPY. Both of them can be used to copy files from the source to destination, but:

COPY: Simple copy of local files into the container
ADD: Will also extract local tar files and supports remote URL. It is less obvious thus it is preferable to use COPY unless you specifically need these features

Creating an application to be containerized

Let’s first create a simple Java application so we can containerize it.

mvn "io.quarkus:quarkus-maven-plugin:create" \
  -DprojectGroupId="com.redhat.developers" \
  -DprojectArtifactId="tutorial-app" \
  -DprojectVersion="1.0-SNAPSHOT" \
  -DclassName="HelloResource" \
  -Dpath="hello"
cd tutorial-app

All the remaining parts of this tutorial assume that you’ll be working inside the project folder that was just created. In this case, tutorial-app.

To let this application be ready to be distributed, let’s package it:

mvn package

Building a Dockerfile

Create a file named Dockerfile.

cat <<EOF >Dockerfile
FROM registry.access.redhat.com/ubi8/openjdk-17

COPY target/quarkus-app/lib/ /deployments/lib/
COPY target/quarkus-app/quarkus-run.jar /deployments/app.jar
COPY target/quarkus-app/app/ /deployments/app/
COPY target/quarkus-app/quarkus/ /deployments/quarkus/

CMD ["java", "-jar", "/deployments/app.jar"]
EOF