switch to staged builds

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20
+FROM golang:1.20 AS builder
 
 WORKDIR /app
 
@@ -6,6 +6,11 @@ ADD go.mod go.sum ./
 RUN go mod download
 
 ADD . ./
-RUN go build -o /backend
+RUN go build -o backend
 
+FROM alpine:latest
+
+RUN apk --no-cache add ca-certificates
+
+COPY --from=builder /app/backend /backend
 CMD ["/backend"]