If you find it acceptable to use Docker to develop/build projects then you probably have different standards than I do. But see my reply on another subthread for some of my issues with ESP-IDF.

I've been using esp-idf (both professionally and as a hobby) for almost a decade by now and I have not once used Docker.

They even provide you with a fully offline SDK. Extract it and open a terminal and build things. No dependencies, no docker, no nothing.

Your criticism regarding their usage of cmake and the kconfig language hold a bit more water. I've never had any issue with the kconfig system (at least not since they've reimplemented the menu system in python instead of whatever it was) but I have hit CMake limitations a few times.

How else do you ensure somewhat consistent CI...? VM images?

A good, hermetic build system. Nix or Bazel or ...