[validation] docker: Keep going after build fail

When building all validation docker images, if one of the image builds
fails, the next image builds are currently skipped.

Keep going through all the build steps, while still exiting with a non
zero return code in case one of the steps fails.

Change-Id: Ib983ec311caaa71c9429280aa9295568380e9387
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>