fix(agent): use per-stage BuildKit cache IDs to avoid parallel write conflict

Author: syscod3

Dockerfile.agent

@@ -3,25 +3,25 @@
 FROM golang:1.25-alpine AS agent-builder
 WORKDIR /workspace
 COPY go.mod go.sum ./
-RUN --mount=type=cache,id=gomod,target=/root/go/pkg/mod go mod download
+RUN --mount=type=cache,id=agent-gomod,target=/root/go/pkg/mod go mod download
 COPY api/ api/
 COPY cmd/agent/ cmd/agent/
 COPY internal/ internal/
 ARG TARGETARCH
-RUN --mount=type=cache,id=gomod,target=/root/go/pkg/mod \
-    --mount=type=cache,id=gobuild,target=/root/.cache/go-build \
+RUN --mount=type=cache,id=agent-gomod,target=/root/go/pkg/mod \
+    --mount=type=cache,id=agent-gobuild,target=/root/.cache/go-build \
     CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} go build -o agent ./cmd/agent
 
 FROM golang:1.25-alpine AS guest-agent-builder
 WORKDIR /workspace
 COPY go.mod go.sum ./
-RUN --mount=type=cache,id=gomod,target=/root/go/pkg/mod go mod download
+RUN --mount=type=cache,id=guest-gomod,target=/root/go/pkg/mod go mod download
 COPY api/ api/
 COPY cmd/guest-agent/ cmd/guest-agent/
 COPY internal/ internal/
 ARG TARGETARCH
-RUN --mount=type=cache,id=gomod,target=/root/go/pkg/mod \
-    --mount=type=cache,id=gobuild,target=/root/.cache/go-build \
+RUN --mount=type=cache,id=guest-gomod,target=/root/go/pkg/mod \
+    --mount=type=cache,id=guest-gobuild,target=/root/.cache/go-build \
     CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH} go build -o guest-agent ./cmd/guest-agent
 
 FROM gcr.io/distroless/static:nonroot AS runtime