Add support for build metadata in version string

To be compatible with SemVer, we should be able to handle versions which
include optional build metadata
([see](https://semver.org/#spec-item-10)). In general, this is already
the case. However, (some) OCI registries don't allow `+` in their tags,
which is used as separator for these build metadata. That's why, the
version has to be sanitised prior to uploading to a registry or after
retrieving it from there.

oci/client.py

@@ -30,6 +30,10 @@
 oci_request_logger = logging.getLogger('oci.client.request_logger')
 oci_request_logger.setLevel(logging.DEBUG)
 
+# additional build metadata as defined in SemVer can be added via `+` to the version
+# however, OCI registries don't allow `+` in tags and use a `META_SEPARATOR` instead
+META_SEPARATOR = '.build-'
+
 
 def _append_b64_padding_if_missing(b64_str: str):
     if b64_str[-1] == '=':
@@ -226,6 +230,8 @@ def manifest_url(self, image_reference: typing.Union[str, om.OciImageReference])
         if not (tag := image_reference.tag):
             raise ValueError(f'{image_reference=} does not seem to contain a tag')
 
+        tag = tag.replace('+', META_SEPARATOR)
+
         return urljoin(
             self.artifact_base_url(image_reference=image_reference),
             'manifests',
@@ -701,7 +707,10 @@ def tags(self, image_reference: str):
             method='GET'
         )
 
-        return res.json()['tags']
+        return [
+            tag.replace(META_SEPARATOR, '+')
+            for tag in res.json()['tags']
+        ]
 
     def has_multiarch(self, image_reference: str) -> bool:
         res = self.head_manifest(