Use Debian 10 for our https-everywhere container image
We are currently using Debian Stretch to build https-everywhere for any platform.
We can update that to Debian 10.
It looks like the https-everywhere build still requires python3.6 while Debian 10 provides version 3.7, so we still need our own build of python3.6. However the update to Debian 10 will allow us to share the python3.6 build with the osx64 and windows builds as they will be using Debian 10 too.