adapt Dockerfile changes for go 1.21 from #491

Dockerfile

@@ -1,5 +1,26 @@
 # Build the manager binary
-FROM registry.access.redhat.com/ubi8/go-toolset:1.20.10 as builder
+
+# BEGIN -- workaround lack of go-toolset for golang 1.21
+
+ARG GOLANG_IMAGE=golang:1.21
+
+ARG GOARCH=amd64
+
+FROM ${GOLANG_IMAGE} AS golang
+FROM registry.access.redhat.com/ubi8/ubi:8.8 AS builder
+
+# Install system dependencies
+RUN dnf upgrade -y && dnf install -y \
+    gcc \
+    make \
+    openssl-devel \
+    && dnf clean all && rm -rf /var/cache/yum
+
+# Install Go
+ENV PATH=/usr/local/go/bin:$PATH
+COPY --from=golang /usr/local/go /usr/local/go
+
+# END -- workaround lack of go-toolset for golang 1.21
 
 WORKDIR /workspace
 # Copy the Go Modules manifests
@@ -14,7 +35,7 @@ COPY pkg/ pkg/
 
 # Build
 USER root
-RUN CGO_ENABLED=1 GOOS=linux GOARCH=amd64 make go-build-for-image
+RUN CGO_ENABLED=1 GOOS=linux GOARCH=${GOARCH} make go-build-for-image
 
 FROM registry.access.redhat.com/ubi8/ubi-minimal:8.8
 WORKDIR /