Dockerfile: use OS and arch from the OS of the build image (#2155)

Dockerfile

@@ -8,7 +8,7 @@ RUN apt-get update && \
 # Build the full app binary
 WORKDIR /app
 COPY . .
-RUN GOOS=linux GOARCH=amd64 CGO_ENABLED=0 make build
+RUN CGO_ENABLED=0 make build
 
 # Copy built binaries to production slim image.
 FROM alpine:3.21 AS final