Add billing service container build

Dockerfile

@@ -0,0 +1,25 @@
+FROM golang:1.25.1-alpine AS builder
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY cmd ./cmd
+COPY internal ./internal
+
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o /out/billing-service ./cmd/billing-service
+
+FROM alpine:3.22
+
+RUN addgroup -S app && adduser -S app -G app
+
+WORKDIR /app
+
+COPY --from=builder /out/billing-service /app/billing-service
+
+USER app
+
+EXPOSE 8081
+
+ENTRYPOINT ["/app/billing-service"]