Build-image: provide ruby 3.0.0 support

[fool]

Currently, using the latest Xenial build image, one is unable to use the released-christmas-day Ruby 3.0.0. When trying to follow the documentation and setting a .ruby-version file with the string 3.0.0 in it, this is what occurs:

6:06:19 PM: Unknown ruby interpreter version (do not know how to handle): 3.0.0.
6:06:19 PM: Attempting ruby version 3.0.0, read from .ruby-version file
6:06:20 PM: 
6:06:20 PM: ** WARNING **
6:06:20 PM: Using custom ruby version 3.0.0, this will slow down the build.
6:06:20 PM: To ensure fast builds, set the RUBY_VERSION environment variable, or .ruby-version file, to an included ruby version.
6:06:20 PM: Included versions: 2.7.1 2.6.2
6:06:20 PM: 
6:06:20 PM: Unknown ruby interpreter version (do not know how to handle): 3.0.0.
6:06:20 PM: Failed to install ruby version '3.0.0'
6:06:20 PM: Build was terminated: Build script returned non-zero exit code: 1
6:06:20 PM: Failing build: Failed to build site

I believe an update to rvm has provided 3.0.0 support as I can run locally:

% rvm list known | grep 3 | grep 0.0
[ruby-]3[.0.0]
% rvm version
rvm 1.29.11 (latest) by Michal Papis, Piotr Kuczynski, Wayne E. Seguin [https://rvm.io]

But in Xenial, an older version does not see that release:

$ rvm list known ; rvm version
[...]
11:24:36 AM: # MRI Rubies
11:24:36 AM: [ruby-]1.8.6[-p420]
11:24:36 AM: [ruby-]1.8.7[-head] # security released on head
11:24:36 AM: [ruby-]1.9.1[-p431]
11:24:36 AM: [ruby-]1.9.2[-p330]
11:24:36 AM: [ruby-]1.9.3[-p551]
11:24:36 AM: [ruby-]2.0.0[-p648]
11:24:36 AM: [ruby-]2.1[.10]
11:24:36 AM: [ruby-]2.2[.10]
11:24:36 AM: [ruby-]2.3[.8]
11:24:36 AM: [ruby-]2.4[.9]
11:24:36 AM: [ruby-]2.5[.7]
11:24:36 AM: [ruby-]2.6[.5]
11:24:36 AM: [ruby-]2.7[.0]
11:24:36 AM: ruby-head
[...]
11:24:36 AM: rvm 1.29.10 (latest) by Michal Papis, Piotr Kuczynski, Wayne E. Seguin [https://rvm.io]

Looking at the release notes for rvm 1.29.11, bumping from 1.29.10 (current version in xenial) to 1.29.11 (latest) should fix this.

[fool]

First requested in community, here: https://community.netlify.com/t/support-for-ruby-3-0/29400
I see about 20 separate attempts to use it since christmas day in our internal logs (cf this internal slack thread which has a link that you can use to assess for the most recent time window at a later point in time)

[vbrown608]

Rebuilding the base image will get us the latest rvm version (it's not pinned), so I suspect we'll get this for free with our next release.