diff --git a/tests/docker/docker.py b/tests/docker/docker.py
index d96ccc9b19..e630aae108 100755
--- a/tests/docker/docker.py
+++ b/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():