Tag Variables
You can use a variable to set the value of a defined tag. When you add the tag to a resource, the variable resolves to the data it represents. You can use tag variables in defined tags and default tags.
Supported Tag Variables
The following tag variables are supported.
${iam.principal.name} The name of the principal that tagged the resource
${iam.principal.type} The type of principal that tagged the resource.
${oci.datetime} The date and time that the tag was created.
Consider the following example:
Operations.CostCenter=" ${iam.principal.name} at ${oci.datetime} "
Operations is the namespace, CostCenter is the tag key, and the tag value contains two tag variables ${iam.principal.name} and ${oci.datetime} . When you add this tag to a resource, the variable resolves to your user name (the name of the principal that applied the tag) and a time date stamp for when you added the tag.
user_name at 2019-06-18T18:00:57.604Z
The variable is replaced with data at the time you apply the tag. If you later edit the tag, the variable is gone and only the data remains. You can edit the tag value in all the ways you would edit any other tag value. To create a tag variable, you must use a specific format.
${<variable>} Type a dollar sign followed by open and close curly brackets. The tag variable goes between the curly brackets. You can use tag variables with other tag variables and with string values. Tag defaults let you specify tags to be applied automatically to all resources, at the time of creation, in a specific compartment. This feature allows you to ensure that appropriate tags are applied at resource creation without requiring the user who is creating the resource to have access to the tag namespaces.
https://docs.cloud.oracle.com/en-us/iaas/Content/Tagging/Tasks/managingtagdefaults.htm

https://blogs.oracle.com/developers/post/working-with-oracle-cloud-infrastructure-custom-compute-images
https://docs.oracle.com/en-us/iaas/Content/Compute/Tasks/managingcustomimages.htm