source/docker: Add regex match for tags

Repositories on Docker Hub often have images we do not want to consider
when determining the "latest" version of an application, such as
non=container images, development/testing versions, etc.  To exclude
these, project sources can now define a `version_re` property that
contains a regular expression.  Images that do not match the expression
will be ignored.

updatebot.py

@@ -79,6 +79,7 @@ class DockerHubSource(BaseSource):
     kind: Literal['docker']
     namespace: str
     repository: str
+    version_re: str = r'^(?P<version>[0-9]+(\.[0-9]+)*(-[0-9]+)?)$'
 
     def get_latest_version(self) -> str:
         session = _get_session()
@@ -89,10 +90,18 @@ class DockerHubSource(BaseSource):
         r = session.get(url)
         data = r.json()
         versions = []
+        regex = re.compile(self.version_re)
         for result in data['results']:
-            if result['name'] == 'latest':
+            m = regex.match(result['name'])
+            if not m:
+                log.debug(
+                    'Skipping tag %s: does not match regex %s',
+                    result['name'],
+                    self.version_re,
+                )
                 continue
-            versions.append((result['last_updated'], result['name']))
+            version = m.groupdict()['version']
+            versions.append((result['last_updated'], version))
         versions.sort()
         return versions[-1][-1]