tests/docker: handle missing encoding keyword for subprocess.check_output

tests/docker: handle missing encoding keyword for subprocess.check_output

This was only added in Python 3.6 and not all the build hosts have
that recent a python3. However we still need to ensure everything is
returns as a unicode string so checks higher up the call chain don't
barf.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
fixup! tests/docker: handle missing encoding keyword for subprocess.check_output