tests/docker: fix update command due to python3 str/bytes distinction

Does this seem convoluted to you? It feels a little complicated to me.

Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <[email protected]>

Author: stsquad

tests/docker/docker.py

@@ -24,7 +24,7 @@
 import re
 import signal
 from tarfile import TarFile, TarInfo
-from io import StringIO
+from io import StringIO, BytesIO
 from shutil import copy, rmtree
 from pwd import getpwuid
 from datetime import datetime, timedelta
@@ -541,13 +541,14 @@ def run(self, args, argv):
 
         # Create a Docker buildfile
         df = StringIO()
-        df.write("FROM %s\n" % args.tag)
-        df.write("ADD . /\n")
-        df.seek(0)
+        df.write(u"FROM %s\n" % args.tag)
+        df.write(u"ADD . /\n")
+
+        df_bytes = BytesIO(bytes(df.getvalue(), "UTF-8"))
 
         df_tar = TarInfo(name="Dockerfile")
-        df_tar.size = len(df.buf)
-        tmp_tar.addfile(df_tar, fileobj=df)
+        df_tar.size = df_bytes.getbuffer().nbytes
+        tmp_tar.addfile(df_tar, fileobj=df_bytes)
 
         tmp_tar.close()