Fix dockerfile for building multi arch images

Author: aleksanderaleksic

Dockerfile

@@ -20,8 +20,8 @@ COPY controllers/ controllers/
 COPY pkg/ pkg/
 
 # Build
-ARG TARGETOS=linux
-ARG TARGETARCH=amd64
+ARG TARGETOS
+ARG TARGETARCH
 RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -a -o manager cmd/manager/main.go
 
 # Use distroless as minimal base image to package the manager binary