Improvements to downloading extensions during build

A deploy today failed in the generated Dockerfile's install-php-extensions step: Imagick's compile downloads a PHP-Parser tarball at build time, received an error page instead (gzip: stdin: not in gzip format → make error 1).

An unchanged redeploy succeeded.

Would it be possible to have either: (a) retry-with-backoff around extension build downloads, and/or (b) pre-bake or layer-cache compiled extensions (at the moment every deploy looks like it currently recompiles Imagick from source, which both slows builds and re-exposes each deploy to upstream download flakiness)?