Linux: engines and apps build in an Ubuntu 26.04 container, glibc 2.43 at most (L1.4)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/linux/Dockerfile

@@ -0,0 +1,20 @@
+# Linux builds on glibc 2.43 run on Ubuntu 26.04 onward.
+FROM ubuntu:26.04@sha256:f144425ff09be612d6d9ad965196e9cdc23dae1f42110a8a11a3e9a8198759f7
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN sed -i 's/^Types: deb$/Types: deb deb-src/' /etc/apt/sources.list.d/ubuntu.sources \
+  && apt-get update \
+  && apt-get build-dep -y libreoffice \
+  && rm -rf /var/lib/apt/lists/*
+RUN apt-get update \
+  && apt-get install -y --no-install-recommends \
+    curl ca-certificates git rsync patch file jq python3 nodejs npm rustup \
+    cmake ninja-build pkg-config nasm \
+    libssl-dev libjitterentropy3-dev libzstd-dev zlib1g-dev \
+    libxkbcommon-dev libxkbcommon-x11-dev libwayland-dev libx11-xcb-dev libxcb1-dev libfontconfig-dev \
+  && rm -rf /var/lib/apt/lists/*
+ENV RUSTUP_HOME=/opt/rustup
+# As rust-toolchain.toml pins it.
+RUN rustup toolchain install 1.96.0 --profile minimal --component rustfmt,clippy \
+  && rustup default 1.96.0 \
+  && chmod -R a+rX /opt/rustup