If you ask the question, “What is a Docker container?”, it turns out that the Linux operating system doesn’t actually have a notion of a container at all. Instead, a Docker container refers to a cobbled together set of Linux technologies, such as cgroups, network namespaces, and union filesystems. However, from the point of view of the end-user, a container is very much a real thing. In particular, it exposes to the user images as an entity, and a command-line tool for pulling down images from a repository and running them.
The Docker container implementation may be built (in the Unix tradition!) with duct table and baling wire, but the user interface is elegant. It’s easy for a new user to get started with Docker once they’ve installed it. Bryan Cantrill points out that the advantage of Docker over container technologies developed in the BSD world is Docker’s notion of images as effectively static binaries that allow developers to think operationally and move faster.
Contrast that with git. The implementation is quite elegant: git represents the data under version control as a hash tree, with pointers into nodes in the tree. Git commands are tree manipulations: adding leaf nodes, moving branches from one part of the tree to another, smooshing nodes together, and so on. (I used Subversion for years and had no idea what was going on under the hood).
On the other hand, the command-line interface that git exposes is a nightmare. It’s so hard to use that trying to build a more usable command-line interface is a full-blown academic research project.
The elegance of a user interface and the elegance of an implementation are orthogonal. One doesn’t necessarily lead to the other.