cmd/cue: move Dockerfile to the main package directory

We only publish a "cuelang/cue" image which only contains the "cue"
binary, so it's rather unnecessary to put the Dockerfile at the top
of the repository. cmd/cue/Dockerfile is more appropriate,
and also avoids needing yet another file at the root of the repository.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ia099a9e7b1709ac3fa80af1ac502dd70d3b2741d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194728
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>

internal/ci/goreleaser/goreleaser.cue

@@ -96,7 +96,7 @@ config: {
 		image_templates: [
 			"docker.io/cuelang/cue:{{ .Version }}-amd64",
 		]
-		dockerfile: "Dockerfile"
+		dockerfile: "cmd/cue/Dockerfile"
 		use:        "buildx"
 		build_flag_templates: [
 			"--platform=linux/amd64",
@@ -114,7 +114,7 @@ config: {
 			"docker.io/cuelang/cue:{{ .Version }}-arm64",
 		]
 		goarch:     "arm64"
-		dockerfile: "Dockerfile"
+		dockerfile: "cmd/cue/Dockerfile"
 		use:        "buildx"
 		build_flag_templates: [
 			"--platform=linux/arm64",