appimage build: pin glibc version in docker image, for reproducibility

fixes #6357

contrib/build-linux/appimage/Dockerfile

@@ -24,6 +24,7 @@ RUN apt-get update -q && \
         libzbar0=0.10+doc-10ubuntu1  \
         libdbus-1-3=1.10.6-1ubuntu3.6 \
         libxkbcommon-x11-0=0.5.0-1ubuntu2.1 \
+        libc6-dev=2.23-0ubuntu11.2 \
         && \
     rm -rf /var/lib/apt/lists/* && \
     apt-get autoremove -y && \