ci: add build stage

Dockerfile

@@ -1,26 +1,12 @@
-FROM golang:1.20-alpine
-
-# Set destination for COPY
+FROM golang:1.21-alpine as builder
 WORKDIR /app
+COPY go.mod ./
+RUN go mod download
 
-# Download Go modules
-# COPY go.mod go.sum ./
-# RUN go mod download
-
-# Copy the source code. Note the slash at the end, as explained in
-# https://docs.docker.com/engine/reference/builder/#copy
 COPY . ./
-
-# Build
 RUN CGO_ENABLED=0 go build -o /finance-api
 
-# Optional:
-# To bind to a TCP port, runtime parameters must be supplied to the docker command.
-# But we can document in the Dockerfile what ports
-# the application is going to listen on by default.
-# https://docs.docker.com/engine/reference/builder/#expose
+FROM gcr.io/distroless/static-debian12
+COPY --from=builder /finance-api .
 EXPOSE 8080
-
-# Run
-# RUN go run main.go serve
 CMD ["/finance-api", "serve", "--http=0.0.0.0:8080"]