Support ARM64 when using Heroku-24 Docker images

[edmorley]

Heroku itself currently runs on AMD64 CPUs, however, some users use our buildpacks locally on machines with ARM64 CPUs (such as M1/M2/M3 MacBooks) with the Heroku base images published to Docker Hub.

As such there have been requests to support the ARM64 architecture, e.g.:
heroku/base-images#194

Starting with Heroku-24, the base images published to Docker Hub are now multi-architecture (AMD64 + ARM64), and our preview Cloud Native Buildpacks support ARM64 when using Heroku-24.

However, until CNBs leave preview there will still be users using our classic buildpacks with our base images from Docker Hub, so it would be ideal if we could add ARM64 support to our classic buildpacks too. This will not only make the images faster to run locally, but also avoid breaking local development workflows if users update to Heroku-24 and miss the mention in the stack upgrade notes about using --platform linux/amd64 to force the architecture back to AMD64.

For example, the buildpack should support:

1. git clone https://github.com/heroku/java-getting-started && cd java-getting-started
2. Add a Dockerfile with the below contents.
3. docker build --tag arm-test --platform linux/arm64 .
4. docker run --rm -e PORT=5001 -p 5001:5001 arm-test
5. curl localhost:5001

FROM heroku/heroku:24-build as build
ENV STACK=heroku-24
COPY --chown=heroku . /app
WORKDIR /app
# This is after the COPY line so buildpack updates are picked up.
RUN mkdir -p /tmp/buildpack /tmp/cache /tmp/env \
  && curl https://buildpack-registry.s3.amazonaws.com/buildpacks/heroku/java.tgz \
    | tar -xz -C /tmp/buildpack
RUN /tmp/buildpack/bin/compile /app /tmp/cache /tmp/env

FROM heroku/heroku:24
COPY --from=build --chown=heroku /app /app
ENV HOME=/app
WORKDIR /app
CMD ["bash", "-c", "for f in .profile.d/*; do source \"${f}\"; done && java -jar target/java-getting-started-1.0.0-SNAPSHOT.jar"]

Currently when I try the above, I get this error (which is expected, since support for ARM64 hasn't yet been added):

 > [build 5/5] RUN /tmp/buildpack/bin/compile /app /tmp/cache /tmp/env:
0.574 -----> Installing OpenJDK 17... done
7.957 -----> Executing Maven
7.959        $ ./mvnw -DskipTests clean dependency:list install
7.961 rosetta error: failed to open elf at /lib64/ld-linux-x86-64.so.2
7.961  
7.963  !     ERROR: Failed to build app with Maven
7.963        We're sorry this build is failing! If you can't find the issue in application code,
7.963        please submit a ticket so we can help: https://help.heroku.com/

cc @Malax