Multi-platform multi-stage build docker images with platform specific files

 While working on multi platform docker images for a FastAPI application with MySQL client libraries, ran into a scenario where the MySQL library files are stored in platform specific paths.

/usr/lib/x86_64-linux-gnu

/usr/lib/aarch64-linux-gnu

As part of multi-stage build, I wanted to do something like

FROM python:${PYTHON_VERSION}-slim

# if x64

COPY --from=builder /usr/lib/x86_64-linux-gnu/libmysql* /usr/lib/x86_64-linux-gnu

# if arm

COPY --from=builder /usr/lib/aarch64-linux-gnu/libmysql* /usr/lib/aarch64-linux-gnu

But there is no conditional platform support at the COPY command level. It is only at the FROM level.

The simple solution would be to have duplicate final image setup.

FROM --platform=amd64 python:${PYTHON_VERSION}-slim

COPY --from=builder /usr/lib/x86_64-linux-gnu/libmysql* /usr/lib/x86_64-linux-gnu
...

FROM --platform=arm64 python:${PYTHON_VERSION}-slim

COPY --from=builder /usr/lib/aarch64-linux-gnu/libmysql* /usr/lib/aarch64-linux-gnu
...

But this would mean duplicating rest of the steps too.

The solution is to create multiple platform specific intermediate images and use them to create the final image.

FROM --platform=amd64 python:${PYTHON_VERSION}-slim as final-intermediate-amd64

COPY --from=builder /usr/lib/x86_64-linux-gnu/libmysql* /usr/lib/x86_64-linux-gnu
COPY --from=builder /usr/lib/x86_64-linux-gnu/libmaria* /usr/lib/x86_64-linux-gnu
COPY --from=builder /usr/lib/x86_64-linux-gnu/*xslt* /usr/lib/x86_64-linux-gnu
COPY --from=builder /usr/lib/x86_64-linux-gnu/*xml* /usr/lib/x86_64-linux-gnu

FROM --platform=arm64 python:${PYTHON_VERSION}-slim as final-intermediate-arm64

COPY --from=builder /usr/lib/aarch64-linux-gnu/libmysql* /usr/lib/aarch64-linux-gnu
COPY --from=builder /usr/lib/aarch64-linux-gnu/libmaria* /usr/lib/aarch64-linux-gnu
COPY --from=builder /usr/lib/aarch64-linux-gnu/*xslt* /usr/lib/aarch64-linux-gnu
COPY --from=builder /usr/lib/aarch64-linux-gnu/*xml* /usr/lib/aarch64-linux-gnu

FROM final-intermediate-${TARGETARCH}

COPY . /code/
COPY requirements.txt /code/requirements.txt
COPY --from=builder /code/.venv /code/.venv
COPY --from=builder /usr/bin/mysql_config /usr/bin

WORKDIR /code

EXPOSE 8000

CMD ["/code/.venv/bin/uvicorn", "--app-dir", "/code/myapp", "server:app", "--proxy-headers"]