Remove unavaliable package dependency

python-is-python3 is unavaliable for Ubuntu 18.04 apparently.
apt-get update before apt-get install should take care of this, but
is removed for the time being.

containers/oneapi/onedev.Dockerfile

@@ -60,7 +60,6 @@ RUN --mount=type=cache,target=/var/cache/apt                            \
         python3                                                         \
         python3-dev                                                     \
         python3-distutils                                               \
-        python-is-python3                                               \
         time                                                            \
         unzip                                                           \
         valgrind                                                        \