Update docker image usage

[lloeki]

What does this PR do?

Make use of the new image versions.

Motivation:

The above.

Additional Notes:

On 1.x-stable, Ruby and JRuby image building should be restricted to those that are not on master.

But really, base images should come from https://github.com/DataDog/images-rb instead.

How to test the change?

CI should pass.

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