Add a C++ friendly docker build file

This takes an existing cross-friendly image and adds cmake, ninja, cargo,
and rust. It also enables dlopen linkage for the compiler.

docker/Dockerfile.cpp-image

@@ -0,0 +1,21 @@
+ARG arch
+FROM ghcr.io/slint-ui/slint/$arch
+ARG arch
+
+RUN apt-get update && \
+    apt-get install --assume-yes gpg wget && \
+    wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /usr/share/keyrings/kitware-archive-keyring.gpg >/dev/null && \
+    echo 'deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ focal main' | tee /etc/apt/sources.list.d/kitware.list >/dev/null && \
+    apt-get update && \
+    apt-get install --assume-yes cmake ninja-build make && \
+    rm -rf /var/lib/apt/lists/*
+
+
+ENV RUSTUP_HOME=/rust
+ENV CARGO_HOME=/cargo 
+ENV PATH=/cargo/bin:/rust/bin:$PATH
+
+RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
+RUN rustup target add $arch
+
+ENV RUST_FONTCONFIG_DLOPEN=on