Add Dockerfile and GHCR publish workflow

.github/workflows/build-image.yaml

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+name: build-image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*.*.*"]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-qemu-action@v3
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Compute tags and labels
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,format=long
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

Dockerfile

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+# Multi-arch image for ts-plug. Cross-compiles Go on BUILDPLATFORM and
+# ships an alpine runtime.
+#
+#   docker buildx build --platform linux/amd64,linux/arm64 \
+#     -t ghcr.io/tailscale/ts-plug:latest -f Dockerfile --push .
+FROM --platform=$BUILDPLATFORM golang:1.26-alpine AS build
+ARG TARGETOS
+ARG TARGETARCH
+# Installed on the builder so we can COPY the bundle to the runtime image
+# without a RUN there — TARGETPLATFORM RUN needs QEMU registered on the host.
+RUN apk add --no-cache ca-certificates
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
+    go build -ldflags='-s -w' -o /out/ts-plug ./cmd/ts-multi-plug
+
+FROM alpine:3.20
+COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=build /out/ts-plug /usr/local/bin/ts-plug
+ENTRYPOINT ["/usr/local/bin/ts-plug"]