docker: replace g++ + make with build-essential

build-essential pulls gcc, g++, make and libc-dev together.
CMake declares LANGUAGES C CXX so it needs a C compiler (cc) too –
individual g++ install can leave gaps that build-essential covers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/docker/Dockerfile.ci-cpp

@@ -9,8 +9,7 @@ RUN apt-get update -qq && \
     apt-get install -y --no-install-recommends \
       nodejs \
       cmake \
-      make \
+      build-essential \
-      g++ \
       git \
     && rm -rf /var/lib/apt/lists/*