Add 'gradle' to sample Dockerfile for building with Gradle

[VanillaSpoon]

Added 'gradle' to sample Dockerfile for building with Gradle, in building-native-image.adoc.

#24727

---

Testing:
Follow the guide from building native image @ Sample Dockerfile for building with Gradle:
Using this dockerfile :)

## Stage 1 : build with maven builder image with native capabilities
FROM quay.io/quarkus/ubi-quarkus-native-image:21.3.1-java11 AS build
COPY --chown=quarkus:quarkus gradlew /code/gradlew
COPY --chown=quarkus:quarkus gradle /code/gradle
COPY --chown=quarkus:quarkus build.gradle /code/
COPY --chown=quarkus:quarkus settings.gradle /code/
COPY --chown=quarkus:quarkus gradle.properties /code/
USER quarkus
WORKDIR /code
COPY src /code/src
RUN gradle -b /code/build.gradle buildNative

## Stage 2 : create the docker final image
FROM quay.io/quarkus/quarkus-micro-image:1.0
WORKDIR /work/
COPY --from=build /code/build/*-runner /work/application
RUN chmod 775 /work
EXPOSE 8080
CMD ["./application", "-Dquarkus.http.host=0.0.0.0"]

[quarkus-bot]

Thanks for your pull request!

The title of your pull request does not follow our editorial rules. Could you have a look?

• title should preferably start with an uppercase character (if it makes sense!)
• title should not start with chore/docs/feat/fix/refactor but be a proper sentence

This message is automatically generated by a bot.