How to execute gradle build before the Docker build on Heroku? - spring-boot

Question

I want to deploy a dockerized Spring-Boot application built with gradle on the Heroku platform when a commit is pushed on github.
I deployed successfully the docker image with CLI, by building the image locally and then deploying it
I deployed successfully "on github push" with the "heroku-18" stack. After each commit, Heroku detects that my project is a gradle one, build it and deploys it with no problem. This method doesn't trigger Docker.
Now, I want to switch to the "container" stack, for Heroku to build my Dockerfile and deploys my app after each commit. The Dockerfile is correctly detected, but because the JAR is not generated, the Dockerfile step fails.
How can I trigger the generation of the JAR on Heroku side for the Dockerfile to be able to copy this JAR to the container ?
Heroku logs
=== Fetching app code
=== Building web (Dockerfile)
Sending build context to Docker daemon 50.69kBStep 1/11 : FROM openjdk:11-jre-slim
11-jre-slim: Pulling from library/openjdk
...
Step 4/11 : ADD build/libs/myapp-1.0-SNAPSHOT.jar /app/myapp.jar
ADD failed: stat /var/lib/docker/tmp/docker-builder545575378/build/libs/myapp-1.0-SNAPSHOT.jar: no such file or directory
heroku.yml
build:
docker:
web: Dockerfile
Dockerfile
FROM openjdk:11-jre-slim
VOLUME /var/log/my-app
ARG JAR_FILE
ADD build/libs/my-app-1.0-SNAPSHOT.jar /app/my-app.jar
RUN chgrp -R 0 /app
RUN chmod -R g+rwX /app
RUN chgrp -R 0 /var/log/my-app
RUN chmod -R g+rwX /var/log/my-app
CMD [ "-jar", "/app/my-app.jar" ]
ENTRYPOINT ["java"]
EXPOSE 8080

Answer 1

Probably you can't do that outside your Dockerfile.
But you can use Docker multi-stage builds like this:
FROM adoptopenjdk/openjdk14-openj9:alpine as build
COPY . /opt/app/src
WORKDIR /opt/app/src
RUN ./gradlew clean bootJar
FROM adoptopenjdk/openjdk14-openj9:alpine-jre
COPY --from=build opt/app/app.jar /opt/app/app.jar
CMD ["java", "-server", "-XX:MaxRAM=256m", "-jar", "/opt/app/app.jar"]