Vendor every dependency, so no build fetches the host's validator (hq to-be 45 D1)

The controller imports mesh-host/validate through a replace onto the forge
that holds it, and every build — the build agent's go build in a fresh
toolchain container, the Dockerfile's go mod download — would have fetched
it through the public proxy and checksum database at build time: a merge
breaking main on the network, the class Phase 1 removes. vendor/ is
committed; go builds from it with nothing fetched, and refuses to build
when it and go.mod disagree, so a pin moved without go mod vendor fails at
once. The Dockerfile copies vendor/ and builds with GOPROXY=off.

Dockerfile

@@ -21,13 +21,15 @@ ARG GO_BASE=golang@sha256:8ac98ca534ac3f51e1f420a1dd2c15e74c75cfa0f23f3ad27eb5d7
 FROM ${GO_BASE} AS build
 WORKDIR /src
 
-# Dependencies first, so a change to the source does not refetch them.
+# **Nothing is fetched** (novox/hq to-be 45 Phase 1): every dependency is in vendor/, committed, so
+# the image builds from this repository alone — the host's validator among them, whose module no
+# public proxy is asked for. Dependencies first, so a change to the source does not re-copy them.
 COPY go.mod go.sum ./
-RUN go mod download
+COPY vendor/ vendor/
 
 COPY . .
 ARG VERSION=development
-RUN CGO_ENABLED=0 go build -trimpath \
+RUN CGO_ENABLED=0 GOFLAGS=-mod=vendor GOPROXY=off go build -trimpath \
         -ldflags "-s -w -X main.version=${VERSION}" \
         -o /mesh-controller ./cmd/mesh-controller