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.

internal/catalogue/genesis_image_test.go

@@ -0,0 +1,29 @@
+package catalogue
+
+import (
+	"os"
+	"regexp"
+	"testing"
+)
+
+// novox/hq issue 223: genesis raises the controller as a container built from this repository's own
+// Dockerfile, with no build arguments — the manifest no longer builds an image, so nothing passes a
+// base in. The Dockerfile's own default must therefore be a Go that builds this module, pinned by
+// digest, and the replacement the manifest's process names must be the container genesis raises.
+func TestGenesisCanBuildTheControllersImageAsItStands(t *testing.T) {
+	raw, err := os.ReadFile("../../Dockerfile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !regexp.MustCompile(`(?m)^ARG GO_BASE=golang@sha256:[0-9a-f]{64}$`).Match(raw) {
+		t.Fatal("the Dockerfile's default Go base is not pinned by digest; genesis builds it with no arguments")
+	}
+	makefile, err := os.ReadFile("../../Makefile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	pin := regexp.MustCompile(`golang@sha256:[0-9a-f]{64}`)
+	if string(pin.Find(raw)) != string(pin.Find(makefile)) {
+		t.Errorf("the Dockerfile and the Makefile build on different Go: %s, %s", pin.Find(raw), pin.Find(makefile))
+	}
+}