Local file:// source for prebuilds?

[jchook]

Similar to this idea: mapbox/node-pre-gyp#101

When building lightweight docker images, musl prebuilds are commonly unavailable, which triggers a node-gyp rebuild.

To speed up CI/CD flow, I would like to prebuild locally, store that in docker layer cache, then have prebuild-install to use that local prebuild as the "download" source for a subsequent npm ci.

Right now it seems that leveldown_binary_host (for example) does not support the file:// protocol (which might be a neat way to support this feature), and I'm left with either tricking prebuild-install into believing it already downloaded an unpublished prebuild, or serving the prebuild over HTTP.

Happy to contribute a PR.

[vweevers]

Related: #114

[jchook]

Seems we could reinstate most of the code removed in 0593bf5 verbatim to solve the issue.

May want an env var to control the local prebuilds path, though perhaps not necessary?

[jchook]

Released in 6.1.0 🎉