The gpg key is in the top level directory for each distribution, using "${DOCKER_EE_URL}/sles/gpg" will attempt to load the gpg key from within the 12.3/<architecture>/<version>/ subdirectory, because of how DOCKER_EE_URL is set before.
In the **Install using the repository** section, the GPG implied location was identified as `<DOCKER-EE-URL>/sles/12.3/<ARCHITECTURE>/stable-<VERSION>/sles/gpg` which does not exist. The correct location is `<DOCKER-EE-URL>/sles/gpg`. I introduce a second variable here called `DOCKER_EE_BASE_URL` which will be used to distinguish between the root level of the directory and aid in correctly locating the gpg key for the `rpm --import` command.