Add additional tags for semver compatible flutter versions

Currently images on ghcr only have specific version tags (eg 3.22.1) and stable/beta tags.
It would be good to also have semver compatible tags for major.minor version in addition to existing tags. This would allow users/CI to specify 3.19 and get the latest release for that version:
eg
```
docker run ghcr.io/cirruslabs/flutter:3.19
```
would fetch and run the latest patch version of 3.19 (this would be 3.19.6 at the time of writing).

Start by tracing the Docker image publishing workflow and the existing generation of specific, stable, and beta tags. Check how Flutter versions are discovered and published, then verify that a major.minor tag resolves to the latest patch release, using the 3.19 example as the acceptance case.

Written by the indexing model from the issue text.