The example images build from the Go the manifest pins

Four example Dockerfiles named golang:1.25 while go.mod requires 1.26;
the control plane's image takes GO_BASE from the manifest and these did
not, so a build that could not fetch a newer toolchain failed at go mod
download (found running the lab through the mesh).

examples/objectstore-provisioner/Dockerfile

@@ -10,7 +10,10 @@
 # The client is copied from the vendor's own image rather than installed from a distribution:
 # `apk add mc` on Alpine installs Midnight Commander, which is a different program with the same
 # name, and the failure would be a provisioner that starts cleanly and cannot do anything.
-FROM golang:1.25-alpine AS build
+# The Go it builds with is the one the manifest pins (build.on GO_BASE), passed by the Makefile and the
+# build machine alike; the default only serves a hand build, and matches go.mod.
+ARG GO_BASE=golang:1.26-alpine
+FROM ${GO_BASE} AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download