Pin the image's Go base in the Dockerfile, which genesis builds as it stands (hq issue 223)

Genesis now raises a process-form controller as a container built from
this repository's Dockerfile with no build arguments (mesh-host
bootstrap, novox/hq issue 223); the manifest builds no image, so nothing
passes the base in. The default was a tag older than go.mod asks for.
It is now the digest the Makefile pins, and a test holds the two equal.

Dockerfile

@@ -1,6 +1,11 @@
-ARG GO_BASE=golang:1.25-alpine
+# The Go it builds with, pinned here because genesis builds this file with no arguments (novox/hq
+# issue 223) — the Makefile passes the same digest. A tag older than go.mod asks for is how
+# `make image` broke once before (issue 146).
+ARG GO_BASE=golang@sha256:8ac98ca534ac3f51e1f420a1dd2c15e74c75cfa0f23f3ad27eb5d7236c349a0c
 # The control plane's image — for genesis and the lab only. The mesh runs the controller as a Go
 # bundle the host starts as a process (module.json; novox/hq issue 213), and builds no image of it.
+# Genesis builds this file and raises it as the container the process replaces on the first push
+# (mesh-host internal/bootstrap, novox/hq issue 223).
 #
 # novox/hq ADR 0006: this image is pinned by digest in the bundle the host carries, fetched on a
 # machine where no mesh exists yet, and run before there is anything to check it against. So it