How do I get zsh 5.xx on ubuntu 18.04

I have a docker container with ubuntu 18.04 on it.

after apt-get update I do:

# apt-get install zsh-beta
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package zsh-beta

Is there a special repository for this I need to add? Is there an apt package for 5.xx?

I get the same response on a non-virtual Ubuntu.

This appears to be required dependency for the "rvm" ruby manager package.

Thanks.

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