Upgrade base image to 2.7.2

We now use Ruby 2.7.2 in most places, and the latest release of nanoc that we're using seems very stable under Ruby 2.7.2.

So after having:

• Upgraded: !1225 (merged)
• Then downgraded: !1238 (merged)

It looks like we should be fine to upgrade again.

This would allow us to use Bundler 2.x everywhere.

CC @axil