Product image selection should only include a slash after the repo if needed

[lfrancke]

Currently a slash is always appended to the repo.

let image = format!("{repo}/{image_base_name}:{product_version}-stackable{stackable_version}",);

While it will probably work if users add a / themselves to the repo it would not be unheard of to cause trouble.
We should therefore only conditionally append the slash.

This behavior is documented in our docs and references this issue so as part of fixing this issue those docs need to be adjusted as well.

• Slash is only appended when repo does not already end with a slash
• Documentation is updated to remove the reference to this issue