Add test for Docker pre-release to stable version update (#14412)

The Docker ecosystem supports updating from pre-release to stable tags
(e.g. 3.7.0a2 to 3.7.0), but this transition had no integration test.
The only pre-release test covers 3.7.0a1 to 3.7.0a2 (pre-release to
pre-release).

Add a minimal fixture containing just the tags needed for the test and
a new context block in both the update_checker_spec and the shared
examples to verify the pre-release to stable path.

Co-authored-by: Hariharan Thavachelvam <164553783+thavaahariharangit@users.noreply.github.com>