Exclude extensions container for FCOS

This excludes all non-base (right now, only extensions) images from
image-references if the build is using fcos, because we currently don't
ship fcos extensions images, and including them would break the
payload.

This also removes the baseOSExtensionsContainerImage from the osimagurl
configmap so the placeholder value doesn't get passed through. (oc won't
rewrite it if it's not in image-references).

Dockerfile

@@ -11,7 +11,14 @@ ARG TAGS=""
 COPY --from=builder /go/src/github.com/openshift/machine-config-operator/instroot.tar /tmp/instroot.tar
 RUN cd / && tar xf /tmp/instroot.tar && rm -f /tmp/instroot.tar
 COPY install /manifests
-RUN if [[ "${TAGS}" == "fcos" ]]; then sed -i 's/rhel-coreos-8/fedora-coreos/g' /manifests/*; \
+
+RUN if [[ "${TAGS}" == "fcos" ]]; then \
+    # comment out non-base/extensions image-references entirely for fcos
+    sed -i '/- name: rhel-coreos-8-/,+3 s/^/#/' /manifests/image-references && \
+    # also remove extensions from the osimageurl configmap (if we don't, oc won't rewrite it, and the placeholder value will survive and get used)
+    sed -i '/baseOSExtensionsContainerImage:/ s/^/#/' /manifests/0000_80_machine-config-operator_05_osimageurl.yaml && \
+    # rewrite image names for fcos 
+    sed -i 's/rhel-coreos-8/fedora-coreos/g' /manifests/* ; \
     elif [[ "${TAGS}" == "scos" ]]; then sed -i 's/rhel-coreos-8/centos-stream-coreos-9/g' /manifests/*; fi && \
     if ! rpm -q util-linux; then yum install -y util-linux && yum clean all && rm -rf /var/cache/yum/*; fi
 COPY templates /etc/mcc/templates