Delete redundant retrievers docker image in docker_images_list.md. Refactor Retrievers related Examples READMEs. Change all of the comps/retrievers/xxx/xxx/Dockerfile path into comps/retrievers/src/Dockerfile. Fix the Examples CI issues of PR opea-project/GenAIComps#1138. Signed-off-by: letonghan <letong.han@intel.com>