Runtime: work around 1s delay added when using twisted reactor's callLater.

Since wxPython4, using wxReactor from non-main thread was producing
exceptions in wxWidget's C++ code. Then reactor.run() was called from
main thread, and runtime's worker was delegating calls to reactor
with callLater(0, callable).

While this worked perfectly with wxReactor, it did introduce an unexplained
1 second delay to each worker call when using nomal linux reactors
(i.e. without wxPython). As a workaround reactor runs in a thread when using
twisted without wxPython

#
# Dockerfile for wxPython3.0 running on python2.7
#

FROM python:2.7-stretch

RUN set -xe \
    && apt-get update \
    && apt-get install -y --no-install-recommends python-wxgtk3.0 python-matplotlib \
    && apt-get install -y --no-install-recommends python-xvfbwrapper xvfb \
    && apt-get clean