Support reproducible builds (except packages)

See docker-library/official-images issue 16044

- `SOURCE_DATE_EPOCH` is added.
  The value is consumed by the build scripts to make the binary reproducible.

- For Alpine, virtual package versions are pinned to "0" to eliminate
  the timestamp-based version numbers that appear in `/etc/apk/world` and `/lib/apk/db/installed`

> [!NOTE]
> The following topics are NOT covered by this commit:
>
> - To reproduce file timestamps in layers, BuildKit has to be executed with
>   `--output type=<TYPE>,rewrite-timestamp=true`.
>   Needs BuildKit v0.13 or later.
>
> - To reproduce the base image by the hash, reproducers may:
>   - modify the `FROM` instruction in Dockerfile manually
>   - or, use the `CONVERT` action of source policies to replace the base image.
>     <https://github.com/moby/buildkit/blob/v0.13.2/docs/build-repro.md>
>
> - To reproduce packages, see the `RUN` instruction hook proposed in
>   moby/buildkit issue 4576
>
Signed-off-by: Akihiro Suda <[email protected]>

Signed-off-by: Akihiro Suda <[email protected]>