update dockerfile to download emulator from github release

[edaniszewski]

fixes #124

No rush on this, as its mostly optimizations/improvements to the deployment/release flow for the emulator (which doesn't change frequently ATM).

This PR does a few things:

• Downloads the built-in emulator plugin (for non-slim images) directly from github releases (gets it from the latest release)
• Removes the emulator binary that lived in this repo and was being added in (no longer needed as per the above)
• Make a slim dockerfile. Before we did this by some no-so-fancy moving around of .dockerignore files. This was both non-intuitive, and with the new changes no longer needed to happen. Now release.dockerfile corresponds to synse-server with emulator, slim.dockerfile corresponds to synse-server without emulator.
• update to build dependency to reduce image size by a decent amount. we kept gcc installed because the gRPC stuff used it. turns out, that it only used some of the dependencies for the gcc install, libstdc++. Now we just keep libstdc++ on the image, which results in ~80Mb saved.
• add some things to dockerignore to further reduce build context/size (small, but it all adds up)
• updated Makefile appropriately

[MatthewHink]

Sorry I though I +1ed this earlier but must have gotten distracted.

[edaniszewski]

thanks -- no worries. I'm going to hold off on merging this until next week for stability. this really shouldn't break anything, but I'd rather not introduce a change into master unless its necessary at this point.

[codecov-io]

Codecov Report

Merging #129 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   88.42%   88.42%           
=======================================
  Files          40       40           
  Lines        1097     1097           
=======================================
  Hits          970      970           
  Misses        127      127

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