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/postgres-provisioner/Dockerfile

@@ -3,7 +3,10 @@
 # Built here so a machine can be given it by the mesh rather than by somebody putting a binary on
 # it. Static and FROM scratch for the same reason the control plane's image is: it is fetched by
 # digest and run on a machine, and everything in it is something a person would have to audit.
-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