Docker client - fixes 64

Fixes #64.

Part of #608, followup is #712.
It's O(N) to build the list and then O(N) to find `unit_name` in it.  It's O(N) to build a set, too, but O(1) to find `unit_name` in it.  And having explicitly typed that out, I realize a generator expression would be better.  It's O(N) to find `unit_name` in the generator expression's iterator and there's no separate build step.

(In other words, this was a minor comment about runtime performance/microoptimization.)