This commit is a first step in gracefully handling existing layers when packing the parts' contents. Specifically, it fixes the existing breakage where priming a file in "/lib" will break a resulting Ubuntu-based rock because on the base "/lib" is a symlink to "/usr/lib", and if that link is broken everything breaks. The updates to support this are: - Enhance oci.Image.add_layer() to take the existing base into account, via a parameter that points to the extracted base (see below); - Update lifecycle code to pass the extracted base forward when packing; - Add a jammy runner to CI, because the new integrated tests need umoci which is only available on jammy. The update to add_layer() is to look for a file's parent dir in the existing base. If it *does* exist, and is a symlink to another dir, add the file as belonging to the symlink target instead. For example: - The prime dir contains "bin/file.txt"; - The base is ubuntu:22.04, which means that there "/bin/" is a symlink to "/usr/bin"; - Instead of adding the file as "bin/file.txt", add it as "usr/bin/file.txt". Note that the original idea of just adding "bin/file.txt" but *not* adding the "bin/" itself seems to be flaky; it works on *some* configurations of the relevant tools but not others (more investigation necessary).