Download SDK index.jsons at docker build.

[isoos]

• SDK search index improvements (tracking issue) #8670
• The cache location is reused from the test, but instead of hand-provided filename, I've opted to use the URL as the template.
• The TTL is there only for the test to re-download the files periodically, so that we catch regressions. (TBD: should we download the beta channel instead in tests?)
• The docker build will download the files, or will fail during the build. However, if Built files are not built, only asserted when the app starts in a container. #8680 lands, we could also assert the presence of the files and not fall back to downloading them.