This is a refactoring in preparation of cleaning up the handling
of key specific subcompletions.
The new `__docker_map_key_of_current_option()` function will be used
instead of the `__docker_map_key_of_current_option()` idiom in the
following commit.
As this function is very specific, checks using it should be executed
before those checking for `$prev`.
This commit just moves the checks without any modification.
Signed-off-by: Harald Albers <github@albersweb.de>
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