Drop Dockerfile.ci, which nothing has built since 2025

It was added in 941e5f0 (2024-09-21) as a cache-less variant, because
the BuildKit cache mounts in the main Dockerfile were causing trouble in
CI, and the workflow pointed at it with `file: Dockerfile.ci`. That line
was removed in 6719538 (2025-02-27, "Switch to using native ARM64
builders"), which returned the build to the default Dockerfile - but the
file itself stayed behind.

Nothing has referenced it since. It had meanwhile drifted from the real
Dockerfile (no RELEASE_BUILD argument, no cache mounts) while Renovate
kept bumping its base image pin, so every one of those bumps was a pull
request for a file that is never built.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -80,7 +80,7 @@ jobs:
           fi
 
           git fetch --no-tags origin main
-          if git diff --name-only origin/main HEAD -- Dockerfile Dockerfile.ci | grep -q .; then
+          if git diff --name-only origin/main HEAD -- Dockerfile | grep -q .; then
             echo 'A Dockerfile changed; the image will be built.'
             echo 'build=true' >> "$GITHUB_OUTPUT"
           else