ci: organize dockerfiles

diff --git a/ci/Dockerfile b/ci/Dockerfile
deleted file mode 120000 (symlink)
index d537b9a..0000000
--- a/ci/Dockerfile
+++ /dev/null
@@ -1 +0,0 @@
-Dockerfile.debian
\ No newline at end of file

diff --git a/ci/Dockerfile.debian b/ci/debian-stable/Dockerfile
similarity index 100%
rename from ci/Dockerfile.debian
rename to ci/debian-stable/Dockerfile

diff --git a/ci/clang-unstable/Dockerfile b/ci/debian-unstable/Dockerfile
similarity index 100%
rename from ci/clang-unstable/Dockerfile
rename to ci/debian-unstable/Dockerfile

diff --git a/ci/clang-unstable/README b/ci/debian-unstable/README
similarity index 100%
rename from ci/clang-unstable/README
rename to ci/debian-unstable/README

diff --git a/ci/Dockerfile.fedora b/ci/fedora/Dockerfile
similarity index 100%
rename from ci/Dockerfile.fedora
rename to ci/fedora/Dockerfile