feat(framework): Add UBI Containerfile for a Flower base image

framework/docker/base/ubi/Containerfile

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+# Copyright 2026 Flower Labs GmbH. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+
+# Multi-stage base image build
+#
+# Building the base image for ARM64 requires installing some additional system dependencies to
+# compile the grpcio package, as they do not provide a pre-built package. However, we don't want
+# the dependencies in the final base image as they are only needed to compile the package.
+# That's why we're creating a multi-stage build. When installing flwr, we use a virtual
+# environment to keep all files in a single isolated directory as described here:
+# https://pythonspeed.com/articles/multi-stage-docker-python/
+
+ARG UBI_IMAGE=registry.redhat.io/ubi9/python-312-minimal
+ARG UBI_VERSION=9.7
+FROM ${UBI_IMAGE}:${UBI_VERSION} AS compile
+USER 0
+
+# Install system dependencies required for compiling grpcio on ARM64
+RUN microdnf install -y --nodocs \
+    gcc \
+    gcc-c++ \
+    libffi-devel \
+    && microdnf clean all \
+    # create virtual env
+    && python -m venv /python/venv
+
+# Make sure we use the virtualenv
+ENV PATH=/python/venv/bin:$PATH
+
+# Install specific version of pip, setuptools and flwr
+ARG PIP_VERSION=26.0.1
+ARG SETUPTOOLS_VERSION=82.0.0
+RUN pip install -U --no-cache-dir \
+    pip==${PIP_VERSION} \
+    setuptools==${SETUPTOOLS_VERSION}
+
+ARG FLWR_VERSION=1.28.0
+ARG FLWR_PACKAGE=flwr
+RUN pip install -U --no-cache-dir ${FLWR_PACKAGE}==${FLWR_VERSION}
+
+FROM ${UBI_IMAGE}:${UBI_VERSION} AS base
+USER 0
+
+# Install runtime dependencies and add non-root user
+RUN microdnf install -y --nodocs \
+    shadow-utils \
+    && useradd \
+    --no-create-home \
+    --home-dir /app \
+    -c "" \
+    --uid 49999 app \
+    && microdnf remove -y shadow-utils \
+    && microdnf clean all \
+    && mkdir -p /app \
+    && chown -R app:app /app
+
+COPY --from=compile --chown=app:app /python/venv /python/venv
+
+# Make sure we use the virtualenv
+ENV PATH=/python/venv/bin:$PATH \
+    # Send stdout and stderr stream directly to the terminal. Ensures that no
+    # output is retained in a buffer if the application crashes.
+    PYTHONUNBUFFERED=1 \
+    # Typically, bytecode is created on the first invocation to speed up following invocation.
+    # However, in Docker we only make a single invocation (when we start the container).
+    # Therefore, we can disable bytecode writing.
+    PYTHONDONTWRITEBYTECODE=1 \
+    # Ensure that python encoding is always UTF-8.
+    PYTHONIOENCODING=UTF-8 \
+    LANG=C.UTF-8 \
+    LC_ALL=C.UTF-8
+
+WORKDIR /app
+USER app
+ENV HOME=/app