Support multiple instances of a capability module

[BSick7]

Summary

Enable Nullstone to allow multiple instances of a capability module to attach to a single app. Currently, this is not possible because the injected environment variables would collide.

How will it work?

1. Nullstone is improved to allow mapping env var names (e.g. to distinguish one POSTGRES_URL from another)
2. Nullstone will require mapping env var names if any of the env vars from all capabilities collide.