BuildKit is the core of docker build, containerizing build steps and providing a directed acyclic graph (DAG) execution engine, allowing parts to proceed in parallel. You may have seen this in action with a multistage docker build. As an independent component, buildkit isn't limited to docker, and several tools have emerged that build on top of it, providing containerized builds with an alternative syntax.
Note: operationally, this does mean that you/your CI infrastructure has to be capable of running or connecting to a buildkit daemon that can spawn and run containers. If your CI is already containerized, this may be an issue.
earthly.dev extends the Dockerfile syntax and mixes in some Makefile syntax (indenting and significant whitespace?). From a certain point of view, it could be seen as Dockerfile++, with native integration for git, conditionals, looping, and outputs, among other things. If your problem with Dockerfiles was too many layers run sequentially, this won't save you.
dagger.io chooses present its api with cue, with steps that get fully resolved as their dependencies are executed. So you get to write your build config in a structured language, but it's a bit unclear from the build definitions on what properties as actually available...