Update Dockerfile to work with MacOS

[joshdentremont]

Link to Github issue or other discussion

#814

What does this PR do?

Add's Noah's suggestion of using variables to chown the workbench directory

What changes were made?

updated dockerfile to use $USER_ID and $GROUP_ID variables

How to test / verify this PR?

Without this PR, building fails on MacOS. Testing should include building on Mac and Linux

Interested Parties

@noahwsmith @DonRichards @mjordan

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Checklist

• Before opening this PR, have you opened an issue explaining what you want to to do?
• Have you included some configuration and/or CSV files useful for testing this PR?
• Have you written unit or integration tests if applicable?
• Does the code added in this PR require a version of Python that is higher than the current minimum version?
• If the changes in this PR require an additional Python library, have you included it in setup.py?
• If the changes in this PR add a new configuration option, have you provided a default for when the option is not present in the .yml file?

[noahwsmith]

Oh- that looks like a much better solution!

[DonRichards]

I agree this looks like a good fix.