Pass yarn install options

[dfreilich]

What happened?

• What were you attempting to do?
  I'd like to provide yarn install with one of the CLI options - specifically, call it with yarn --registry <MY_REGISTRY>.

• What did you expect to happen?
  I was expecting the yarn buildpack to respect an ENV variable that would allow me to set yarn options

• What was the actual behavior? Please provide log output, if possible.
  There is no such env variable that allows configuring yarn install behavior.

Build Configuration

• What platform (pack, kpack, tekton buildpacks plugin, etc.) are you
  using? Please include a version.
  This is with pack v0.20.0

• What buildpacks are you using? Please include versions.
  yarn install @v0.4.0

• What builder are you using? If custom, can you provide the output from pack inspect-builder <builder>?
  paketobuildpacks/builder:base

• Can you provide a sample app or relevant configuration (buildpack.yml,
  nginx.conf, etc.)?

[dfreilich]

In this case, there's a bug in yarn that makes it not accept environment variables (yarnpkg/yarn#2091), so passing it in as a yarn install option would be really helpful

[arjun024]

I think we can take it as a feature request where users can request custom extra flags, something like BP_YARN_INSTALL_FLAGS="--X Y --Z A". Given there's a way around it via yarnrc, we'll take it as a low-prio feature request

[fg-j]

Hey @paketo-buildpacks/nodejs-maintainers. I'm curious if you think this feature request is still valuable given that .npmrc and .yarnrc service bindings are now available.

[ryanmoran]

I'm wondering if @dfreilich can speak to whether or not the issue is resolved by #292.

[dfreilich]

I think it could still be useful. Currently, our system writes a .yarnrc file in all cases and passes it along to pack, so #292 doesn't quite help in that sense, and the advantage of this would be that we'd save on creating that file for every single build we do.