switch to slim build of debian 9

[danrubins]

It looks like we can make a lighter-weight image by switching to a slim build. Also might as well update from debian 8 to 9.

[abernix]

Thanks for opening this! Probably worth discussing a bit first... notably:

• It definitely might look we could make it slimmer, and I think it would be very reasonable to add an additional -slim tag if you're willing to do the work to add it, but I definitely chose the non-slim image intentionally because this image was intended as a replacement for users who were using the existing meteorhacks/meteord image.

  Those users leveraged all sorts of things which are only available in the more full image and often need to be able leverage more advanced compilation which (I'm almost certain) is only available through the more hefty image.

• I'd like to keep this on Debian 8 until Debian 9 actually enters Long-Term Support, which doesn't even happen for almost another year: https://wiki.debian.org/LTS/. Debian 8 is under LTS until June 30, 2020, but once Debian 9 goes into LTS, we have 6 months to make that transition. At that point, I think it makes more sense to bump the version.

That said, thoughts on the separate tag?