Change circleci config to allow renovatebot to handle updates of the docker images

[rpl]

In #371 we are migrating the CI job to use the new cimg/node images, unlike the older docker images the cimg/node ones are tight to specific nodejs versions (e.g. nodejs 12.22 instead of any nodejs 12 version) and so the config may need to be manually updated more often than it used to be.

Renovatebot should be able to detect updates to the docker images and automatically submit pull requests, but it is not unlikely that by interpolating the node version into the actualy cimg/node image renovatebot may not be able to detect and handle the docker images updates.

This issue goal is to change the config to allow renovatebot to detect and handle these updates, but deferred as a separate followup so that we can confirm if that is actually necessary.