Before this change, specifying the `--pull` flag without a value, could
result in the flag after it, or the positional argument to be used as
value.
This patch makes sure that the value is an expected value;
docker create --pull --rm hello-world
docker: invalid pull option: '--rm': must be one of "always", "missing" or "never".
docker run --pull --rm hello-world
docker: invalid pull option: '--rm': must be one of "always", "missing" or "never".
docker run --pull hello-world
docker: invalid pull option: 'hello-world': must be one of "always", "missing" or "never".
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
190dac77bd