docker: Use $VERSION_CODENAME to setup apt source (#18)

Signed-off-by: Mikael Arguedas <mikael.arguedas@gmail.com>

.github/docker/ros-core/Dockerfile

@@ -18,8 +18,9 @@ RUN apt-get update && apt-get install -q -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 # setup sources.list
-RUN curl -sSL https://ros.packages.techfak.net/gpg.key -o /etc/apt/keyrings/ros-one-keyring.gpg && \
-    echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/ros-one-keyring.gpg] https://ros.packages.techfak.net jammy main" | tee /etc/apt/sources.list.d/ros1.list
+RUN curl -sSL https://ros.packages.techfak.net/gpg.key -o /etc/apt/keyrings/ros-one-keyring.gpg \
+    && . /etc/os-release \
+    && echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/ros-one-keyring.gpg] https://ros.packages.techfak.net $VERSION_CODENAME main" | tee /etc/apt/sources.list.d/ros1.list
 
 # setup environment
 ENV LANG=C.UTF-8