ci: Automatically run black on every code commit

black is a tool to reformat Python source code into a standard format.
On the CI we will use it in --check mode so that it complains if it
spots any incorrectly formatted code. Let's enforce this for every
source code commit.

For this to work well, let's also automatically build a new version of
the ci container image for use in the pipeline. If the pipeline succeeds
and is running on main branch, then promote the built ci image to
"latest".

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>