The main benefit docker introduced, was leading developers to at least consider "configuration injection" and "stateless installs" ("12 factor apps").

If upstream supplies a decent docker image, chances are that means the package is more amenable to scripting and running in a chroot/jail/container - and documents it dependencies somewhat.

That said, snaphotting the "state" of your container/jail can be nice. Recently I used the official ruby images, and could build a custom image to use with our self-hosted gitlab (with built-in docker registry) that a) got a standard Debian based ruby image and applied updates, and b) built freetds for connecting to mssql.

Now I can quickly update that base image as needed, while the CI jobs testing our rails projects "only" need a "bundle install" before running tests.

And the scripts are largely reusable across heterogeneous ruby versions (yes I hope we can get all projects up on a recent ruby..).