resolving docker build issue

src/rocker/core.py

@@ -136,7 +136,7 @@ def get_docker_client():
 def docker_build(docker_client = None, output_callback = None, **kwargs):
     image_id = None
     build_success = False
-    build_output = []
+    error_message = []
 
     if not docker_client:
         docker_client = get_docker_client()
@@ -149,17 +149,20 @@ def docker_build(docker_client = None, output_callback = None, **kwargs):
             continue
         if output_callback is not None:
             output_callback(output)
-        build_output.append(output)
 
         match = re.match(r'Successfully built ([a-z0-9]{12})', output)
         if match:
             image_id = match.group(1)
             build_success = True
+        elif "error" in output.lower():
+            error_message.append(output)
+
 
     if build_success:
         return image_id
     else:
-        raise Exception("Build failed: "+"\n".join(build_output))
+        error_msg = "\n".join(error_message)
+        raise Exception(f"Build failed:\n{error_msg}")
 
 
 class SIGWINCHPassthrough(object):