Add additional tags for semver compatible flutter versions

[jsidebottom-restech]

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).