gitlab-ci: Fix pipeline with latest software and explicit images

• The errors produced by b4 has changed in v12.x and our tests were based on b4 v0.9.x, hence modify the test expectations accordingly.
• Specify the images used in different stages explicitly, since the previous defaults are missing some required packages.

Signed-off-by: Senthil Kumaran S senthil.kumaran@linaro.org