Allow Docker image to be used with a non-root user

[samherrmann]

For demonstration purposes, consider running the license_finder container as follows:

$ docker run -it --rm -v $(pwd):/scan --user $(id -u):$(id -g)  licensefinder/license_finder

Inside of the container....

$ /LicenseFinder/bin/license_finder
/usr/bin/env: ‘ruby’: Permission denied

It would be helpful if the image could be used with a non-root user.

[cf-gitbot]

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