Switch to signing with cosign instead of skopeo.

Fixes #1

images/Dockerfile_buildah

@@ -3,4 +3,9 @@ FROM quay.io/buildah/upstream:latest
 # Add skopeo.
 RUN dnf install -y skopeo \
     --exclude container-selinux \
-    --enablerepo=updates-testing
+    --enablerepo=updates-testing
+
+# Add wget, use it to install cosign.
+RUN dnf install -y wget && \
+    wget https://github.com/sigstore/cosign/releases/download/v2.1.1/cosign-2.1.1.x86_64.rpm && \
+    rpm -ivh cosign*.rpm