Add support for mold in build image

Dockerfile-ubuntu2204

@@ -51,6 +51,9 @@ RUN cp -prv /usr/lib/llvm-15/lib/libc++abi.so.1.0 /usr/lib/llvm-15/lib/libc++abi
 # EDIT: no longer needed!
 # RUN cp -prv /usr/lib/llvm-14/lib/libunwind.so.1.0 /usr/lib/llvm-14/lib/libunwind.so
 
+# Install mold (even faster than gold) (linker)
+RUN apt install -y mold
+
 COPY switch-to-latest-clang /usr/local/bin/switch-to-latest-clang
 
 RUN chmod +x /usr/local/bin/switch-to-latest-clang