Create automation to produce image signature config map

[dlystra]

In order to update an air gapped OCP cluster, an image signature config map is required. Due to an internet access requirement, the image signature config map creation will need to be automated during the collection process.

[usrbinkat]

Example snippet:

OCP_RELEASE_NUMBER=4.6.3
ARCHITECTURE=x86_64
DIGEST="$(oc adm release info quay.io/openshift-release-dev/ocp-release:${OCP_RELEASE_NUMBER}-${ARCHITECTURE} | sed -n 's/Pull From: .*@//p')"
DIGEST_ALGO="${DIGEST%%:*}"
DIGEST_ENCODED="${DIGEST#*:}"
SIGNATURE_BASE64=$(curl -s "https://mirror.openshift.com/pub/openshift-v4/signatures/openshift/release/${DIGEST_ALGO}=${DIGEST_ENCODED}/signature-1" | base64 -w0 && echo)
cat >checksum-${OCP_RELEASE_NUMBER}.yaml <<EOF
apiVersion: v1
kind: ConfigMap
metadata:
  name: release-image-${OCP_RELEASE_NUMBER}
  namespace: openshift-config-managed
  labels:
    release.openshift.io/verification-signatures: ""
binaryData:
  ${DIGEST_ALGO}-${DIGEST_ENCODED}: ${SIGNATURE_BASE64}
EOF