Update Binder build config (option 1)

[ptitzler]

This PR updates the Binder build file removing git related directories to hide the fact that the work directory itself is a cloned git repository. See #962 for details and other approaches that didn't seem to yield the desired result.

Closes #962

Developer's Certificate of Origin 1.1

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[ptitzler]

• Confirmed that the deployment on mybinder yields the expected results: https://mybinder.org/v2/gh/ptitzler/elyra/i962-option-2?urlpath=lab/tree/binder-demo
• Confirmed that a local build/deployment using jupyter-repo2docker --ref i962-option-2 https://github.com/ptitzler/elyra yields the expected results

Only caveat is that a user can navigate to the parent folder using the File Browser, which exposes the Elyra source code. This won't do any harm though. The only way to avoid this would be to restructure the content of the home directory as part of the postBuild, which I'd rather not do.

[kevin-bates]

These changes look good. I just had some comments regarding how others looking at this file in the future would know what these changes are about.

[kevin-bates]

Excellent - thank you Patrick. There's a minor typo, but I approve these changes.

[ptitzler]

Excellent - thank you Patrick. There's a minor typo, but I approve these changes.

Fixed and thank you!

[ptitzler]

@lresende?

[akchinSTC]

LGTM