chore: handle nightly version numbers

Here, we make it possible to add a "-nightly" suffix to the package
version. This suffix will find its way to the Docker image tags. Thus,
the nightly branch will have different image tags. This will resolve
some confusion, as image tags are currently identical in nightly and
master.

tutordiscovery/__about__.py

@@ -1,2 +1,8 @@
 __version__ = "15.0.0"
 
+# Handle version suffix for nightly, just like tutor core.
+__version_suffix__ = ""
+
+if __version_suffix__:
+    __version__ += "-" + __version_suffix__
+