It is not required, but not ending the path with a slash is bad practice since the result depends on the directory already existing or not.
If the destination directory already exists, there does not need to be a slash and the file is copied inside the directory.
If the destination directory does not exist and there is no slash at the end, the file is copied to a file ‘default’
If the destination directory does not exist and there is a slash at the end, the directory is created and the file is copied into it.
In this case the copying seems to go well (by luck).
However,
Since there are no suggestions after “did you mean one of the following?”, the file is not in the correct place. It looks like the drives are mapped incorrectly to the docker container.
You could start a backup from within Docker (discourse backup) and see where it ends up on your host filesystem.