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

@@ -169,8 +169,10 @@ class OciRoutes:
     def __init__(
         self,
         base_api_url_lookup: typing.Callable[[str], str]=base_api_url,
+        sanitise_tags: bool=False,
     ):
         self.base_api_url_lookup = base_api_url_lookup
+        self.sanitise_tags = sanitise_tags
 
     def artifact_base_url(
         self,
@@ -226,6 +228,9 @@ 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')
 
+        if self.sanitise_tags:
+            tag = oci.util.sanitise_tag(tag=tag)
+
         return urljoin(
             self.artifact_base_url(image_reference=image_reference),
             'manifests',
@@ -251,15 +256,22 @@ def __init__(
         disable_tls_validation=False,
         timeout_seconds: int=None,
         session: requests.Session=None,
+        sanitise_tags: bool=False,
     ):
         self.credentials_lookup = credentials_lookup
         self.token_cache = OauthTokenCache()
         if not session:
             self.session = requests.Session()
         else:
             self.session = session
-        self.routes = routes
+        if routes:
+            self.routes = routes
+        else:
+            self.routes = OciRoutes(
+                sanitise_tags=sanitise_tags,
+            )
         self.disable_tls_validation = disable_tls_validation
+        self.sanitise_tags = sanitise_tags
 
         if timeout_seconds:
             timeout_seconds = int(timeout_seconds)
@@ -701,7 +713,15 @@ def tags(self, image_reference: str):
             method='GET'
         )
 
-        return res.json()['tags']
+        tags = res.json()['tags']
+
+        if self.sanitise_tags:
+            tags = [
+                oci.util.desanitise_tag(tag=tag)
+                for tag in tags
+            ]
+
+        return tags
 
     def has_multiarch(self, image_reference: str) -> bool:
         res = self.head_manifest(

oci/util.py

@@ -4,6 +4,8 @@
 import threading
 import typing
 
+META_SEPARATOR = '.build-'
+
 
 def normalise_image_reference(image_reference: str):
   if not isinstance(image_reference, str):
@@ -42,6 +44,27 @@ def urljoin(*parts):
     return '/'.join([first] + middle + [last])
 
 
+def sanitise_tag(tag: str) -> str:
+    '''
+    Additional build metadata as defined in SemVer can be added via `+` to the version. However,
+    OCI registries don't support `+` as tag character, which is why it has to be sanitised, for
+    example using `META_SEPARATOR`.
+    '''
+    sanitised_tag = tag.replace('+', META_SEPARATOR)
+
+    return sanitised_tag
+
+
+def desanitise_tag(tag: str) -> str:
+    '''
+    This function reverts the sanitisation of the `sanitise_tag` function, which allows processing
+    the tag the same way as prior to uploading to the OCI registry using `sanitise_tag`.
+    '''
+    desanitised_tag = tag.replace(META_SEPARATOR, '+')
+
+    return desanitised_tag
+
+
 class _TeeFilelikeProxy:
     '''
     Takes a filelike object (which may be a non-seekable stream) and patches its