Change proxy_cache_key to use header token as part of its key
Solution 1:
@Alexy Ten posted the link to the correct location in the docs, but just to make it easier for people in the future:
$http_name arbitrary request header field; the last part of a variable name is the field name converted to lower case with dashes replaced by underscores
So in your case it would look like: proxy_cache_key $http_x_authtoken.