CI modifications

[Ido David (ido-namely)]

1. Ensure latest tag is only used for final releases.
2. Publishes releases only for "legal" version names.
   i.e ${GRPC_VERSION}_${BUILD_VERSION} for official releases or
   ${GRPC_VERSION}_${BUILD_VERSION}-${RC_VERSION} for pre releases.
   e.g v1.37_2, v1.38_0-rc3, etc.
3. Add success conditions to CI steps (don't continue if previous step failed).
4. Fix image name for PR/master builds (use short commit sha).
5. Local builds will generate images with the name ${GRPC_VERSION}_local by default.
6. Fix build status badge in readme.