Currently, the container tag for 99cloud/skyline in the documentation is
uified as "latest" for the corresponding latest branch, which is
inconsistent with the inconsistent with the version branches.
Update 99cloud/skyline container tag from "latest" to "2023.2" in
doc/source/install/docker-install-ubuntu.rst to align with the branch.
Change-Id: If2e93c1fa50f6745e7b670f01d212bd8872c2b5f