This feature allows one to specify an https URL for any of the
digest-file options, resulting in an HTTP PUT to the provided
URL. This could for example be a (pre-signed) URL to S3 or GCS.
Currently the final digest is only written to the local filesystem,
which disappears and is not accessible when Kaniko is run in a
managed container service like AWS ECS.
By supporting https a single implementation supports all storage
services, without the need for special code for S3, GCS, etc..
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