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

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+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"]

modules/build-agent/README.md

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+# build-agent
+
+The mesh's build machine as a role every machine can hold (novox/hq ADR 0190). It holds the node seat
+`node-build-agent`: every holder pulls one build at a time from the role's one work queue when it is
+idle, so a tier of many images is built by as many machines as hold the seat and are online, and a
+machine that is off builds nothing and blocks nothing. The controller asks the role, never a machine;
+the outcome names the machine that built it.
+
+What a holding machine needs is what the builder always needed, said here once: a container runtime
+(the socket is mounted), the artifact store and the package registry as provisions, a workspace, and
+the bus credential. The code is `cmd/mesh-builder` in the mesh-controller repository, compiled from
+that repository's main (`build.artifacts[].context`); this module ships the packaging.
+
+Assign it to every machine with a container runtime. It replaces `builder`, the one-holder form of the
+same thing; retire that once this is assigned where it was.

modules/build-agent/module.json

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+{
+  "module": "build-agent",
+  "version": "1",
+  "capabilities": [
+    "container-runtime"
+  ],
+  "claims": [
+    {
+      "name": "node-build-agent",
+      "scope": "node"
+    }
+  ],
+  "requires": [
+    "artifact-store",
+    "npm-package-registry"
+  ],
+  "binds": {
+    "npm-package-registry": "${dir:mesh-state}/package-registry.json"
+  },
+  "secrets": {
+    "npm-package-registry": "${dir:mesh-state}/package-registry.secret"
+  },
+  "own-secrets": {
+    "broker": "${dir:mesh-state}/broker"
+  },
+  "resources": [
+    {
+      "id": "mesh-state",
+      "type": "directory",
+      "mode": "0700",
+      "place": "mesh"
+    },
+    {
+      "id": "workspace",
+      "type": "directory",
+      "mode": "0700"
+    },
+    {
+      "id": "agent-env",
+      "type": "file",
+      "path": "${dir:mesh-state}/build-agent.env",
+      "mode": "0600",
+      "content": "MESH_BROKER_FILE=/run/mesh/broker\nMESH_NODE=${machine:name}\nMESH_REGISTRY=${bound:artifact-store:at}:${bound:artifact-store:port}\nMESH_PACKAGE_BINDING=/run/mesh/package-registry.json\nMESH_NPM_TOKEN_FILE=/run/mesh/package-registry.secret\nMESH_WORKSPACE=${dir:workspace}\n"
+    },
+    {
+      "id": "server",
+      "type": "container",
+      "name": "mesh-build-agent",
+      "artifact": "server",
+      "env-file": [
+        "${dir:mesh-state}/build-agent.env"
+      ],
+      "volumes": [
+        "${dir:mesh-state}:/run/mesh:ro",
+        "${dir:workspace}:${dir:workspace}",
+        "/var/run/docker.sock:/var/run/docker.sock"
+      ],
+      "restart-on": [
+        "agent-env"
+      ],
+      "network": "host"
+    }
+  ],
+  "build": {
+    "artifacts": [
+      {
+        "name": "server",
+        "kind": "image",
+        "from": "Dockerfile",
+        "context": {
+          "seat": "git",
+          "repository": "novox/mesh-controller",
+          "ref": "main"
+        }
+      }
+    ],
+    "on": [
+      {
+        "arg": "GO_BASE",
+        "image": "golang@sha256:8ac98ca534ac3f51e1f420a1dd2c15e74c75cfa0f23f3ad27eb5d7236c349a0c"
+      },
+      {
+        "arg": "ALPINE_BASE",
+        "image": "alpine@sha256:d9e853e87e55526f6b2917df91a2115c36dd7c696a35be12163d44e6e2a4b6bc"
+      }
+    ]
+  }
+}