Dockerfile.rhel: Use a RHEL-version-agnostic filename

So we don't have a nominally-rhel7 Dockerfile building RHEL 8 images
and also so we don't need to manually bump this filename when we bump
RHEL versions.  We've had a RHEL 8 base since a4044f4 (Updating
Dockerfile.rhel7 baseimages to mach ocp-build-data config, 2020-08-24, #442).

Once consumers have updated, we can remove the backwards-compat
symlink.

Author: wking

Dockerfile.rhel

@@ -0,0 +1,14 @@
+FROM registry.svc.ci.openshift.org/ocp/builder:rhel-8-golang-openshift-4.6 AS builder
+WORKDIR /go/src/github.com/openshift/cluster-version-operator
+COPY . .
+RUN hack/build-go.sh; \
+    mkdir -p /tmp/build; \
+    cp _output/linux/$(go env GOARCH)/cluster-version-operator /tmp/build/cluster-version-operator
+
+FROM registry.svc.ci.openshift.org/ocp/4.6:base
+COPY --from=builder /tmp/build/cluster-version-operator /usr/bin/
+COPY install /manifests
+COPY vendor/github.com/openshift/api/config/v1/0000_00_cluster-version-operator_01_clusterversion.crd.yaml /manifests/
+COPY vendor/github.com/openshift/api/config/v1/0000_00_cluster-version-operator_01_clusteroperator.crd.yaml /manifests/
+COPY bootstrap /bootstrap
+ENTRYPOINT ["/usr/bin/cluster-version-operator"]

Dockerfile.rhel7

@@ -0,0 +1 @@
+Dockerfile.rhel