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.
Fixes a number of links that were pointing to the wrong location for the linux post-installation steps.
* Move /engine/installation to /install * Add Docker for IBM Cloud to the matrix