Figure out Ubuntu codename in Dockerfile

ci/ubuntu-gcc11-x64.dockerfile

@@ -20,7 +20,8 @@ RUN apt-get update \
 
 # Cmake ppa
 RUN 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
-RUN 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
+RUN export OS_SHORTNAME=$(lsb_release -a | grep -i "codename:" | grep -o "\w\+$")
+RUN echo "deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ $OS_SHORTNAME main" | tee /etc/apt/sources.list.d/kitware.list >/dev/null
 
 # gcc ppa
 RUN add-apt-repository ppa:ubuntu-toolchain-r/test