Why not keep the same docker image throughout then lifecycle? E.g. merge to dev branch, trigger ci (build image at this point), maybe deploy to a test environme

Why not keep the same docker image throughout then lifecycle? E.g. merge to dev branch, trigger ci (build image at this point), maybe deploy to a test environment, run more tests, then deploy to prod. No chance of packages changing since the image isn't rebuilt. Of course if not using docker, a lock file (i.e. actual dependency resolution) would seem essential for reproducibility.