docker.py/build: support binary files in --extra-files

Read the --extra-files in binary mode to avoid encoding errors.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/docker/docker.py

@@ -56,15 +56,19 @@ class EngineEnum(enum.IntEnum):
 
 USE_ENGINE = EngineEnum.AUTO
 
+def _bytes_checksum(bytes):
+    """Calculate a digest string unique to the text content"""
+    return hashlib.sha1(bytes).hexdigest()
+
 def _text_checksum(text):
     """Calculate a digest string unique to the text content"""
-    return hashlib.sha1(text.encode('utf-8')).hexdigest()
+    return _bytes_checksum(text.encode('utf-8'))
 
 def _read_dockerfile(path):
     return open(path, 'rt', encoding='utf-8').read()
 
 def _file_checksum(filename):
-    return _text_checksum(_read_dockerfile(filename))
+    return _bytes_checksum(open(filename, 'rb').read())
 
 
 def _guess_engine_command():