Delete ci/docker, assuming it is not used. (flutter#50103)

I tried grepping for `docker` and couldn't find anything relevant: ![Screenshot 2024-01-26 at 2 52 59â�¯PM](https://github.com/flutter/engine/assets/168174/be75b61f-3c27-4475-8481-e096dff4197a) (Maybe this is "secretly" used by another repo, but there is no way for me to tell that)
3akat · Jan 26, 2024 · 859b80a · 859b80a
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diff --git a/ci/docker/build/Dockerfile b/ci/docker/build/Dockerfile
diff --git a/ci/docker/build/README.md b/ci/docker/build/README.md
diff --git a/ci/docker/build/build_docker.sh b/ci/docker/build/build_docker.sh
diff --git a/ci/docker/build/engine_gclient b/ci/docker/build/engine_gclient
diff --git a/ci/docker/build/push_docker.sh b/ci/docker/build/push_docker.sh
diff --git a/ci/docker/windows_build/README.md b/ci/docker/windows_build/README.md