tests/docker: make executable an optional argument to "update"

We're going to extend the abilities of the command shortly.

Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20210512102051.12134-4-alex.bennee@linaro.org>
2 files changed