R minimal docker

[dselivanov]

This adds docker image based on Alpine linux. It is really small - 62 mb uncompressed (compared to debian-based 750mb from rocker project!) and it seems works just fine in most of the cases. However:

• keep in mind that MUSL is less common than glibc, so sometimes issues arise. For example - https://github.com/dmlc/xgboost/issues/5131.
• we don't add alternative malloc in this PR (this is for further investigation - see for example https://github.com/microsoft/mimalloc). So this issue is valid to some extend. Not a big deal though - Rserve forks and kills processes, so memory fragmentation doesn't hurt too much.

Fixes #115

[codecov[bot]]

Codecov Report

Merging #116 into dev will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #116   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files          27       27           
  Lines        1303     1303           
=======================================
  Hits         1211     1211           
  Misses         92       92

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