LanguageTool releases can contain mismatched versions

[Erikvl87]

LanguageTool could create a (patch) release that potentially doesn't affect the standalone version. In that case they won't always update the version of the release inside the languagetool-standalone directory.
See languagetool-org/languagetool#3659

This breaks the current build of the Dockerfile since it assumes the same version in the languagtool-standalone directory. This blocked the Docker release of 5.1.1 and 5.1.2.

To prevent checking the changelog on every release if it affects the standalone version, the Dockerfile needs to be updated to be able to fall back on whatever version is being shipped inside that folder.