Fix Read-Only filesystem permission issue for log file

repo2docker/buildpacks/repo2docker-entrypoint

@@ -30,17 +30,21 @@ def main():
     #  2. current working directory: "."
     #  3. default temp directory for the OS (e.g. /tmp for linux)
     log_dirs = [".", tempfile.gettempdir()]
+    log_file = None
     if "REPO_DIR" in os.environ:
         log_dirs.insert(0, os.environ["REPO_DIR"])
     for d in log_dirs:
         log_path = os.path.join(d, ".jupyter-server-log.txt")
         try:
             log_file = open(log_path, "ab")
-        except PermissionError:
+        except Exception:
             continue
         else:
             # success
             break
+    # raise Exception if log_file could not be set
+    if log_file is None:
+        raise Exception("Could not open '.jupyter-server-log.txt' log file " )
 
     # build the command
     # like `exec "$@"`