If your goal is just to update a plugin without updating the Discourse version, then simply use the Docker Manager’s upgrade console accessed from the online dashboard and hit the relevant upgrade button for the specific plugin, e.g.:
https://your-site/admin/upgrade
Caveat #1: I believe this will only work with public assets, e.g. open source plugins. For private assets
accessed by deploy keys embedded in the build script you may need to rebuild the entire project.
Caveat #2: A lot of plugin updates are deployed to add compatibility for the latest build of Discourse. In most cases it makes sense to upgrade both to the latest version, because some upgrades can break compatibility with older versions of Discourse, but depends on what you are doing.