Dockerfile: qualify docker.io registry

Other container runtimes can use Dockerfiles, but complain if the
registry is unqualified. It's a good practice to qualify this so
it is not implied.

Dockerfile

@@ -1,5 +1,5 @@
-ARG from_builder=golang:1.24-alpine3.21
-ARG from=alpine:3.21
+ARG from_builder=docker.io/golang:1.24-alpine3.21
+ARG from=docker.io/alpine:3.21
 
 ARG BUILDPLATFORM