--- a/testing/docker/recipes/run-task
+++ b/testing/docker/recipes/run-task
@@ -23,25 +23,25 @@ import os
 import pwd
 import re
 import subprocess
 import sys
 
 
 def print_line(prefix, m):
     now = datetime.datetime.utcnow()
-    print('[%s %sZ] %s' % (prefix, now.isoformat(), m), end='')
+    print(b'[%s %sZ] %s' % (prefix, now.isoformat(), m), end=b'')
 
 
 def run_and_prefix_output(prefix, args):
     """Runs a process and prefixes its output with the time.
 
     Returns the process exit code.
     """
-    print_line(prefix, 'executing %s\n' % args)
+    print_line(prefix, b'executing %s\n' % args)
 
     # Note: TaskCluster's stdin is a TTY. This attribute is lost
     # when we pass sys.stdin to the invoked process. If we cared
     # to preserve stdin as a TTY, we could make this work. But until
     # someone needs it, don't bother.
     p = subprocess.Popen(args,
                          # Disable buffering because we want to receive output
                          # as it is generated so timestamps in logs are
@@ -52,17 +52,17 @@ def run_and_prefix_output(prefix, args):
                          stdin=sys.stdin.fileno(),
                          cwd='/',
                          # So \r in progress bars are rendered as multiple
                          # lines, preserving progress indicators.
                          universal_newlines=True)
 
     while True:
         data = p.stdout.readline()
-        if data == '':
+        if data == b'':
             break
 
         print_line(prefix, data)
 
     return p.wait()
 
 
 def vcs_checkout(args):
@@ -107,17 +107,17 @@ def vcs_checkout(args):
          b'--template', b'{node}'],
         cwd=args.vcs_checkout)
 
     assert re.match('^[a-f0-9]{40}$', revision)
     os.environ['GECKO_HEAD_REV'] = revision
 
 
 def main(args):
-    print_line('setup', 'run-task started\n')
+    print_line(b'setup', b'run-task started\n')
 
     if os.getuid() != 0:
         print('assertion failed: not running as root')
         return 1
 
     # Arguments up to '--' are ours. After are for the main task
     # to be executed.
     try:
@@ -167,34 +167,34 @@ def main(args):
         # And that it is owned by the appropriate user/group.
         os.chown('/home/worker/hg-shared', uid, gid)
         os.chown(os.path.dirname(checkout), uid, gid)
 
     # Drop permissions to requested user.
     # This code is modeled after what `sudo` was observed to do in a Docker
     # container. We do not bother calling setrlimit() because containers have
     # their own limits.
-    print_line('setup', 'running as %s:%s\n' % (args.user, args.group))
+    print_line(b'setup', b'running as %s:%s\n' % (args.user, args.group))
     os.setgroups(gids)
     os.umask(022)
     os.setresgid(gid, gid, gid)
     os.setresuid(uid, uid, uid)
 
     # Checkout the repository, setting the GECKO_HEAD_REV to the current
     # revision hash. Revision hashes have priority over symbolic revisions. We
     # disallow running tasks with symbolic revisions unless they have been
     # resolved by a checkout.
     if checkout:
         vcs_checkout(args)
     elif not os.environ.get('GECKO_HEAD_REV') and \
             os.environ.get('GECKO_HEAD_REF'):
         print('task should be defined in terms of non-symbolic revision')
         return 1
 
-    return run_and_prefix_output('task', task_args)
+    return run_and_prefix_output(b'task', task_args)
 
 
 if __name__ == '__main__':
     # Unbuffer stdio.
     sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)
     sys.stderr = os.fdopen(sys.stderr.fileno(), 'w', 0)
 
     sys.exit(main(sys.argv[1:]))