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>

renovate.json

@@ -25,7 +25,7 @@
 			"platformAutomerge": false
 		},
 		{
-			"description": "The release image's base (Dockerfile and Dockerfile.ci). Gated by ci.yml's docker-build job, which builds the image exactly as Publish does but with `push: false`, and which only runs when a Dockerfile actually changed.",
+			"description": "The release image's base (Dockerfile). Gated by ci.yml's docker-build job, which builds the image exactly as Publish does but with `push: false`, and which only runs when the Dockerfile actually changed.",
 			"matchManagers": [
 				"dockerfile"
 			],