Add semver support when using git tags (#69)

* Add semver tag support

* Allow tag_names to be a fallback for tag_semver

README.md

@@ -187,3 +187,16 @@ with:
   password: ${{ secrets.DOCKER_PASSWORD }}
   tag_names: true
 ```
+
+### tag_semver
+Use `tag_semver` when you want to push tags using the semver syntax by their git name (e.g. `refs/tags/v1.2.3`). This will push four
+docker tags: `1.2.3`, `1.2` and `1`. A prefix 'v' will automatically be removed.
+> CAUTION: Images produced by this feature can be override by branches with the same name - without a way to restore.
+
+```yaml
+with:
+  name: myDocker/repository
+  username: ${{ secrets.DOCKER_USERNAME }}
+  password: ${{ secrets.DOCKER_PASSWORD }}
+  tag_semver: true
+```