Previously, the preview server Docker image was based on Debian 9
(stretch).
This commit upgrades the preview server Docker image to Debian 10
(buster) and also upgrades all dependencies to latest versions
(including upgrading Node.js from v10 to v12).
(The GCE VM running the preview server Docker container was also
upgraded from Debian 9 to 10 on 2020-04-27.)
---
Other changes:
- Pinned the installed version of `curl` to make the `aio-health-check`
and `aio-verify-setup` checks (which use `curl`) more deterministic.
- Dropped the `*-backports` Debian repositories, since they are no
longer needed. The `*-backports` repositories were introduced in
593fe5ed259d5b118d41e2b961b462e922762700 to install `nginx` from, but
became obsolete in 2f1a862b831f44b9b923dcc52f94af69d3b40aa6, which
switched to installing `nginx` from the regular repositories again.
- Added `vim` to the list of installed dependencies (for convenience
during debugging).
PR Close#36837
e73daa3736