[docker] Schema compile fails in ubuntu xenial container

# make
[  1%] Generating schema_enum.h, schema_enum.c
Run python script '/qpid-dispatch/src/schema_c.py':ValueError: Invalid schema qdrouter.json: the JSON object must be str, not 'bytes'
Traceback (most recent call last):
  File "/qpid-dispatch/python/qpid_dispatch_internal/management/qdrouter.py", line 45, in __init__
    super(QdSchema, self).__init__(**json.loads(qd_schema, **JSON_LOAD_KWARGS))
  File "/usr/lib/python3.5/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'bytes'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/qpid-dispatch/build/tests/run.py", line 132, in <module>
    run_path(sys.argv[0], run_name="__main__")
  File "/qpid-dispatch/build/tests/run.py", line 116, in run_path
    runpy.run_path(file_path, run_name=run_name)
  File "/usr/lib/python3.5/runpy.py", line 254, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "/usr/lib/python3.5/runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "/usr/lib/python3.5/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/qpid-dispatch/src/schema_c.py", line 111, in <module>
    Generator()
  File "/qpid-dispatch/src/schema_c.py", line 58, in __init__
    self.schema = QdSchema()
  File "/qpid-dispatch/python/qpid_dispatch_internal/management/qdrouter.py", line 47, in __init__
    raise ValueError("Invalid schema qdrouter.json: %s" % e)
ValueError: Invalid schema qdrouter.json: the JSON object must be str, not 'bytes'
src/CMakeFiles/qpid-dispatch.dir/build.make:71: recipe for target 'src/schema_enum.h' failed
make[2]: *** [src/schema_enum.h] Error 1
CMakeFiles/Makefile2:981: recipe for target 'src/CMakeFiles/qpid-dispatch.dir/all' failed
make[1]: *** [src/CMakeFiles/qpid-dispatch.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2