How to: 🔨 Reference other targets using +
Use `+` to reference other targets and create complex build inter-dependencies.
Examples

* Same directory (same Earthfile)

  ```earthly
  BUILD +some-target
  FROM +some-target
  COPY +some-target/my-artifact ./
  ```

* Other directories

  ```earthly
  BUILD ./some/local/path+some-target
  FROM ./some/local/path+some-target
  COPY ./some/local/path+some-target/my-artifact ./
  ```

* Other repositories

  ```earthly
  BUILD github.com/someone/someproject:v1.2.3+some-target
  FROM github.com/someone/someproject:v1.2.3+some-target
  COPY github.com/someone/someproject:v1.2.3+some-target/my-artifact ./
  ```