* [web] tweak Jenkins pipeline following move to faster CI VMs
The webpack build time has been reduced by about 30%. Adjust the sleep
statement for delaying the webpack build accordingly.
Push the production docker image layers from the parallel steps already.
Use a shared "scratch" tag to avoid generating cruft per branch/build.
* [web] avoid sharing the cypress/downloads folder in CI
GitOrigin-RevId: 52fdf420ec04fd26e3823ff9fca8c52f7a7764d7