build-agent: the build machine as a node role every machine can hold (hq ADR 0190)

The builder's manifest with one change that matters: it claims node-build-agent, a node seat, so it
is assignable to every machine with a container runtime, and every holder pulls one build at a time
from the role's one work queue. A tier of many images is then built by as many machines as hold the
seat and are online. The builder module stays until this is assigned where it was; then it goes.

modules/build-agent/Dockerfile

@@ -0,0 +1,25 @@
+ARG GO_BASE
+ARG ALPINE_BASE
+# build-agent's image: the build machine itself, compiled into a container (novox/hq ADR 0190).
+#
+# **The source is not vendored here.** builder's actual code — cmd/mesh-builder, internal/builder,
+# internal/catalogue — lives in the mesh-controller repository, the same control plane it is one
+# half of. This module ships the packaging, not a second copy of the source, so the build context
+# is the mesh-controller repository root (declared under build.artifacts[].context), and this
+# Dockerfile compiles ./cmd/mesh-builder from it — the same shape route-proxy already uses for the
+# same reason.
+FROM ${GO_BASE} AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -o /mesh-builder ./cmd/mesh-builder
+
+# Unlike mesh-controller's own FROM scratch (ADR 0006: nothing to audit but one binary), the build
+# machine's whole job is shelling out to git and docker — it needs a real userland to do that in,
+# not a second copy of either tool vendored into this image. apk installs both from the base's own
+# packages, not fetched on its own at build time.
+FROM ${ALPINE_BASE}
+RUN apk add --no-cache docker-cli git
+COPY --from=build /mesh-builder /usr/local/bin/mesh-builder
+ENTRYPOINT ["/usr/local/bin/mesh-builder"]