That's sorta true, but the layered-userspace-filesystem support is a pretty big feature that Docker pioneered Linux support for.
Did userspace filesystems/layered filesystem-in-a-box implementations exist before? Sure: from ZFS to squashfs to tar, all the components were around.
But Docker's popularity is due to its integration; not its novelty. By bundling a filesystem-in-a-box abstraction layer with varying degrees of native OS support into something like a mountable artifact/image, hiding the image caching mechanics behind a nice CLI and image-configuration file format (love 'em or hate 'em, Dockerfiles are a phenomenal example of minimalism and convenience: add some meta commands, and everything else is just shell), and integrating all that with the privacy/isolation (namespaces/chroots) and resource management (cgroups/quotas) stuff, Docker made a really, really powerful model of thinking about "containers" as a single concept.
The thought model is Docker's big achievement: other competitors may unseat Docker eventually, but the concept of "container" as a single, unitary thing that's almost like a VM image will stick around, and the more piecemeal understanding of how containers work will be largely unnecessary, and thus save a lot of time for a lot of people.