feat(ci): add Docker image for CI build

.github/ci-image/Dockerfile

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+# Pre-baked CI image for KTailctl.
+#
+# Bundles everything the "git_hooks", "lint", and "native_build" jobs in
+# .github/workflows/ci.yml used to set up from scratch on every single run:
+# the Kitware and KDE Neon apt repos, all packages from scripts/neon-deps.sh,
+# the lint toolchain (clang-tools, clazy, clang-tidy), and cppcheck/iwyu
+# built from source. Baking this once removes that (repeated, slow) setup
+# from every CI job.
+#
+# Built and pushed to ghcr.io by .github/workflows/ci-image.yml, on a
+# schedule and whenever this file or the scripts it depends on change.
+
+# Pinned separately from the FROM line (rather than e.g. ubuntu:latest) so
+# Dependabot can bump it, and so it's a one-line change once KDE Neon
+# publishes packages for a newer codename. Can't go past 24.04 (noble) yet:
+# archive.neon.kde.org/user has no dists/ for 26.04's "resolute" codename.
+ARG UBUNTU_VERSION=24.04
+FROM docker.io/library/ubuntu:${UBUNTU_VERSION}
+
+ENV DEBIAN_FRONTEND=noninteractive
+# Picked up by scripts/ci/apt-add-llvm-repo.sh, build-cppcheck.sh and
+# build-iwyu.sh below (each defaults to the same values if unset).
+ENV LLVM_VERSION=22
+ENV CPPCHECK_VERSION=2.20.0
+
+# Tools the setup scripts below need: wget/gnupg for the repo signing keys,
+# lsb-release for `lsb_release -c -s`, git for cloning cppcheck/iwyu, sudo
+# because the scripts are shared with the (non-root) native CI/dev setup.
+RUN apt-get update -yq \
+    && apt-get install -yq --no-install-recommends \
+        ca-certificates \
+        git \
+        gnupg \
+        lsb-release \
+        sudo \
+        wget \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /opt/ktailctl-src
+COPY scripts/ scripts/
+
+# Mirrors the steps that used to run in ci.yml on every job: add the repos,
+# install the build/runtime dependencies plus the lint tool packages, then
+# build cppcheck and include-what-you-use from source.
+RUN ./scripts/ci/apt-add-kitware-repo.sh \
+    && ./scripts/ci/apt-add-kde-neon-repo.sh \
+    && ./scripts/ci/apt-add-llvm-repo.sh \
+    # apt-add-kde-neon-repo.sh pins the whole Neon origin to priority 900,
+    # which would otherwise also shadow Kitware's much newer cmake with
+    # Neon's own (older) packaged cmake -- too old for this project's
+    # cmake_minimum_required(VERSION 3.31). Force cmake specifically back
+    # to Kitware.
+    && printf 'Package: cmake cmake-data\nPin: origin apt.kitware.com\nPin-Priority: 1001\n' \
+        > /etc/apt/preferences.d/kitware-cmake.pref \
+    && apt-get update -yq \
+    && ./scripts/neon-deps.sh \
+    && apt-get install -yq --no-install-recommends \
+        "clang-tidy-${LLVM_VERSION}" \
+        clang-tools \
+        clazy \
+        libpcre3-dev \
+    && ./scripts/ci/build-cppcheck.sh \
+    && ./scripts/ci/build-iwyu.sh \
+    && rm -rf /tmp/cppcheck /tmp/iwyu /var/lib/apt/lists/* \
+    && cmake --version
+
+# actions/checkout leaves the workspace owned by a different uid than the
+# container runs as; git otherwise refuses to touch it ("dubious ownership").
+RUN git config --system --add safe.directory '*'
+
+# Nothing at container-runtime needs root -- all package installs above run
+# at image-build time -- so run as a non-root user by default. The ubuntu:*
+# base image already ships a passwordless "ubuntu" user (uid/gid 1000) for
+# exactly this purpose.
+USER ubuntu
+WORKDIR /home/ubuntu

.github/workflows/ci-image.yml

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+---
+name: CI Image
+"on":
+  push:
+    branches:
+      - main
+    paths: &ci-image-paths
+      - .github/ci-image/Dockerfile
+      - .github/workflows/ci-image.yml
+      - scripts/neon-deps.sh
+      - scripts/ci/apt-add-kde-neon-repo.sh
+      - scripts/ci/apt-add-kitware-repo.sh
+      - scripts/ci/apt-add-llvm-repo.sh
+      - scripts/ci/build-cppcheck.sh
+      - scripts/ci/build-iwyu.sh
+  # So a PR that touches the image can publish and use it (via the same
+  # :latest tag) before it's merged -- ci.yml's jobs would otherwise have
+  # no image to pull. Same-repo PRs only: forked PRs get a read-only
+  # GITHUB_TOKEN and the GHCR push would just fail.
+  pull_request:
+    branches:
+      - main
+    paths: *ci-image-paths
+  schedule:
+    # Weekly, so the image picks up upstream package/security updates even
+    # when nothing in this repo changes.
+    - cron: "17 3 * * 1"
+  workflow_dispatch: {}
+jobs:
+  build:
+    name: Build & publish CI image
+    runs-on: ubuntu-latest
+    if: >-
+      github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+      - name: Log in to GHCR
+        uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
+      - name: Build and push
+        uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6
+        with:
+          context: .
+          file: .github/ci-image/Dockerfile
+          push: true
+          tags: |
+            ghcr.io/f-koehler/ktailctl-ci:latest
+            ghcr.io/f-koehler/ktailctl-ci:${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max