Aliasing is something done at a web-server level, not application. It's nothing to do with JIRA. (Also, it doesn't work most of the time, most applications work with a single url location, and aliases just land on that and use the core url from that point on)
All you can do about changing urls in Cloud is https://confluence.atlassian.com/display/CLOUDKB/Changing+the+Cloud+Instance+URL+Address+or+Migrating+to+a+New+Cloud+Instance
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