update docs for packaging in a container

README.md

@@ -229,6 +229,10 @@ container to avoid screwing up your main system:
 ```sh
 podman run -it --name pyceo-packaging -v "$PWD":"$PWD" -w "$PWD" debian:buster bash
 ```
+**Important**: Make sure to use a container image for the same distribution which you're packaging.
+For example, if you're creating a package for bullseye, you should be using the debian:bullseye
+Docker image (this is because the virtualenv symlinks python to the OS' version of python).
+
 Here are some of the prerequisites you'll need to build the deb files:
 ```sh
 apt install devscripts debhelper git-buildpackage