[image-build] Add traceback on failure

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/image_build.py

@@ -4,6 +4,7 @@ import copy
 import os
 import time
 from kobo import shortcuts
+import traceback
 
 from pungi.util import get_variant_data, resolve_git_url, makedirs, get_mtime, get_file_size
 from pungi.phases.base import PhaseBase
@@ -153,6 +154,8 @@ class CreateImageBuildThread(WorkerThread):
                 msg = ('[FAIL] image-build for variant %s (%s) failed, but going on anyway.\n%s'
                        % (variant.uid, subvariant, exc))
                 self.pool.log_info(msg)
+                tb = traceback.format_exc()
+                self.pool.log_debug(tb)
 
     def worker(self, num, compose, variant, subvariant, cmd):
         arches = cmd["image_conf"]["image-build"]['arches'].split(',')