cmd-generate-release-meta: inject oci-image key in release metadata

Take the digest pullspec for the base OS bootable container and put it
in the new `oci-image` field in the release metadata.

Part of coreos/fedora-coreos-tracker#1823.

src/cmd-generate-release-meta

@@ -142,6 +142,11 @@ def append_build(out, input_):
     # build the architectures dict
     arch_dict = {"media": {}}
     ensure_dup(input_, arch_dict, "ostree-commit", "commit")
+
+    # we always expect base-oscontainer so error out if missing
+    assert 'base-oscontainer' in input_
+    arch_dict['oci-image'] = populate_oci_image(input_['buildid'], input_['base-oscontainer'])
+
     platforms = ["aliyun", "applehv", "aws", "azure", "azurestack", "digitalocean", "exoscale", "gcp", "hyperv", "ibmcloud", "kubevirt", "metal", "nutanix", "openstack", "powervs", "qemu", "virtualbox", "vmware", "vultr", "qemu-secex"]
     for platform in platforms:
         if input_.get("images", {}).get(platform, None) is not None: