Add PPC Ubuntu Docker image and use it on CI

.github/workflows/docker.yml

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+on:
+  push:
+    branches:
+      - master
+    paths:
+      - 'docker/**'
+  pull_request_target:
+    paths:
+      - 'docker/**'
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-and-push:
+    name: Build & Push Docker Image
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v2
+        with:
+          platforms: linux/amd64,linux/arm64
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.DOCKER_TOKEN }}
+
+      - name: Build and push multi-arch image
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: ./docker/ubuntu.Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true  # ${{ github.ref == 'refs/heads/master' }}
+          tags: ghcr.io/learning-process/ppc-ubuntu:latest

docker/ubuntu.Dockerfile

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+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN set -e \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends \
+    build-essential \
+    ca-certificates curl wget gnupg lsb-release software-properties-common \
+    python3 python3-pip \
+    ninja-build cmake make \
+    ccache \
+    valgrind \
+    libmpich-dev mpich \
+    openmpi-bin openmpi-common libopenmpi-dev \
+    libomp-dev \
+    gcc-14 g++-14 \
+    gcovr \
+ && wget -q https://apt.llvm.org/llvm.sh \
+ && chmod +x llvm.sh \
+ && ./llvm.sh 20 all \
+ && rm llvm.sh \
+ && apt-get clean \
+ && rm -rf /var/lib/apt/lists/*
+
+ENV CC=gcc-14 CXX=g++-14
+
+CMD ["bash"]