use proper name for kaniko cache names

[mojtaba-esk]

While building an image using kaniko, the following warning can be seen in the logs:

WARN[0005] Error uploading layer to cache: getting tag for destination: repository can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-./`: bb7a5c161d9535a225a7fa67e665b4d8305204a82bff3c75cb925cf578f20b7e:24h 

It seems like using colon : for the layer cache name is invalid. This issue is created to keep track of it. It might help to optimize the building process.