tests/docker: add NOFETCH env variable for testing

tests/docker: add NOFETCH env variable for testing

Testing non-auto built docker containers (i.e. custom built compilers)
is a bit fiddly as you couldn't continue a build with a previously
locally built container. While you can play games with REGISTRY its
simpler to allow a NOFETCH that will go through the cached build
process when you run the tests.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241023113406.1284676-3-alex.bennee@linaro.org>