Fix image name for Workspace

What does this MR do and why?

Fixes the name of the container image we're providing for Workspaces.

We want to use gitlab-docs/workspaces from our registry here: https://gitlab.com/gitlab-org/gitlab-docs/container_registry

How to validate

You can verify this image exists from your local machine like this:

$ docker manifest inspect registry.gitlab.com/gitlab-org/gitlab-docs/workspace

This returns some information about the image. If you run that command against what we had previously, like this, you'll get an error:

$ docker manifest inspect registry.gitlab.com/gitlab-org/gitlab-docs:workspace
no such manifest: registry.gitlab.com/gitlab-org/gitlab-docs:workspace

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