ci: build Linux releases on Ubuntu 20.04 for compatibility

Change the Linux build platform from Ubuntu 22.04 to Ubuntu 20.04 so release packages target glibc 2.31 and install on Ubuntu 20.04.

docker/linux-build/Dockerfile

@@ -1,15 +1,16 @@
 # syntax=docker/dockerfile:1.7
 
-ARG BASE_IMAGE=nvidia/cuda:12.6.3-devel-ubuntu22.04
+ARG BASE_IMAGE=nvidia/cuda:12.6.3-devel-ubuntu20.04
 FROM ${BASE_IMAGE}
 
 ARG TARGETARCH
+ARG CMAKE_VERSION=3.31.6
 ARG XMAKE_VERSION=3.0.9
 ARG RUST_VERSION=1.92.0
 ARG RUSTUP_VERSION=1.28.2
 
 LABEL org.opencontainers.image.title="CrossDesk Linux build environment" \
-      org.opencontainers.image.description="Ubuntu 22.04 build environment for CrossDesk" \
+      org.opencontainers.image.description="Ubuntu 20.04 compatibility build environment for CrossDesk" \
       org.opencontainers.image.source="https://github.com/kunkundi/crossdesk" \
       org.opencontainers.image.licenses="GPL-3.0-only"
 
@@ -22,6 +23,8 @@ ENV DEBIAN_FRONTEND=noninteractive \
     XMAKE_GLOBALDIR=/data \
     XMAKE_ROOT=y \
     CUDA_PATH=/usr/local/cuda \
+    CC=/usr/bin/gcc-10 \
+    CXX=/usr/bin/g++-10 \
     PATH=/root/.local/bin:/opt/cargo/bin:/usr/local/bin:${PATH}
 
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
@@ -35,13 +38,14 @@ RUN apt-get update \
         build-essential \
         ca-certificates \
         ccache \
-        cmake \
         curl \
         dbus \
         dpkg-dev \
         file \
         flex \
         gettext \
+        gcc-10 \
+        g++-10 \
         git \
         libasound2-dev \
         libdbus-1-dev \
@@ -52,11 +56,9 @@ RUN apt-get update \
         libfreetype6-dev \
         libgl1-mesa-dev \
         libmount-dev \
-        libpipewire-0.3-dev \
         libpulse-dev \
         libreadline-dev \
         libsndio-dev \
-        libspa-0.2-dev \
         libssl-dev \
         libtool \
         libx11-dev \
@@ -95,6 +97,22 @@ RUN apt-get update \
         zlib1g-dev \
     && rm -rf /var/lib/apt/lists/*
 
+# Slint 1.17 requires CMake 3.21 or newer, while Ubuntu 20.04 provides 3.16.
+# PyPI publishes pinned manylinux2014 wheels for both amd64 and arm64.
+RUN python3 -m pip install --no-cache-dir "cmake==${CMAKE_VERSION}" \
+    && cmake --version | grep -F "cmake version ${CMAKE_VERSION}"
+
+# Ubuntu 20.04 only ships PipeWire 0.2. Install the Ubuntu 22.04-era 0.3.48
+# headers as a build-only SDK. CrossDesk loads the host's PipeWire 0.3 runtime
+# with dlopen(), so no PipeWire shared library is copied into release packages.
+COPY docker/linux-build/install-pipewire-sdk.sh \
+    /usr/local/bin/install-crossdesk-pipewire-sdk
+
+RUN chmod 0755 /usr/local/bin/install-crossdesk-pipewire-sdk \
+    && install-crossdesk-pipewire-sdk \
+    && grep -F "#define PW_MICRO 48" \
+        /opt/crossdesk-pipewire-sdk/include/pipewire-0.3/pipewire/version.h
+
 RUN export GITHUB_ACTIONS=true \
     && curl -fsSL https://xmake.io/shget.text \
         | bash -s -- "v${XMAKE_VERSION}" \
@@ -136,7 +154,8 @@ COPY submodules/minirtc/thirdparty ./submodules/minirtc/thirdparty
 # CrossDesk's exact dependency graph. Source files are intentionally excluded
 # from the Docker context; source-only changes therefore reuse this layer.
 RUN xmake repo -u --root \
-    && xmake f -m release --USE_CUDA=true --USE_WAYLAND=true --root -y \
+    && xmake f -m release --USE_CUDA=true --USE_WAYLAND=true \
+        --cc=gcc-10 --cxx=g++-10 --root -y \
     && rm -rf /data/.xmake/cache /tmp/crossdesk-dependencies \
     && mkdir -p /workspace