ci(cflite): rename .clusterfuzzlite/Containerfile -> Dockerfile (#46)

## Summary

- ClusterFuzzLite calls plain `docker build`; that step fails with
`failed to read dockerfile: open Dockerfile: no such file or directory`
  because the build context only has `Containerfile` (the Podman
  spelling). Rename to `Dockerfile` (contents unchanged) so the build
  step succeeds.
- Last of the four lanes flagged in #41. The other three
  (`a2ml-validate`, `k9-validate`, `hypatia-scan`) were already fixed in
  111a929 and bc7453e on `main`.

## Test plan

- [x] Diff is a pure rename (`git mv`)
- [ ] CI fuzz job builds the image past the dockerfile-resolution step
- [ ] Issue #41 ticked off after this lands

Refs #41

Author: hyperpolymath