Use self.log rather than logging module directly

Using logging.info or .debug directly sets up the root logger,
and after that call every single message is duplicated! This
showed up only when using the dataverse provider.

With this, no messages are duplicated

repo2docker/contentproviders/doi.py

@@ -58,7 +58,7 @@ class DoiProvider(ContentProvider):
                 # Reraise any other errors because if the DOI service is down (or
                 # we hit a rate limit) we don't want to silently continue to the
                 # default Git provider as this leads to a misleading error.
-                logging.error(f"DOI {doi} does not resolve: {e}")
+                self.log.error(f"DOI {doi} does not resolve: {e}")
                 raise
             return resp.url
         else:
@@ -70,7 +70,7 @@ class DoiProvider(ContentProvider):
         # file related to a record
         file_url = deep_get(file_ref, host["download"])
         fname = deep_get(file_ref, host["filename"])
-        logging.debug(f"Downloading file {file_url} as {fname}\n")
+        self.log.debug(f"Downloading file {file_url} as {fname}\n")
 
         yield f"Requesting {file_url}\n"
         resp = self._request(file_url, stream=True)