Expose BUNDLE_WITHOUT as ARG to enable building test and production images

[tmaier]

I think the old fly.io images also allowed this.

This allows me to build the image with BUNDLE_WITHOUT=development
This would be the image to use in the CI pipeline for testing.
After all tests were successful, one would build the production image with BUNDLE_WITHOUT=development:test.

ARG BUNDLE_WITHOUT=development:test
ENV BUNDLE_WITHOUT ${BUNDLE_WITHOUT}

[tmaier]

I see now that there is the option --ci, which does exactly this.