Update release process documentation

What does this MR do and why?

Adds documentation and a code comment about a change in Docker image naming.

For 17.5 and beyond, we are only creating one Dockerized version of each release. Prior to this, we were creating two (one with Google search, and one with Lunr search). Going forward we'll just be creating one with Lunr. This change was introduced in !5098 (merged).

This change means that we'll need to reference Docker versions using an updated path when we update latest.Dockerfile as part of the release process.

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