Add Dockerfile to deploy the relay server

Multi-stage build: compiles with CGO disabled in golang:1.25-alpine,
runs the static binary in a plain alpine:3.20 image. Defaults to
"share relay" on port 9009; override with extra args at `docker run`.

Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.25-alpine AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -o /out/share .
+
+FROM alpine:3.20
+COPY --from=build /out/share /usr/local/bin/share
+EXPOSE 9009
+ENTRYPOINT ["share"]
+CMD ["relay"]