add Dockerfile for Alpine Linux image

[hefnawi]

Please follow up discussion in the main issue here: #534

[codecov-io]

Codecov Report

Merging #664 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #664      +/-   ##
==========================================
- Coverage   39.46%   39.16%   -0.31%     
==========================================
  Files          18       18              
  Lines        3963     3963              
  Branches      653      653              
==========================================
- Hits         1564     1552      -12     
- Misses       2292     2302      +10     
- Partials      107      109       +2

Impacted Files	Coverage Δ
elasticluster/gc3pie_config.py	14.28% <0%> (-2.39%)	⬇️
elasticluster/utils.py	48.68% <0%> (-2.2%)	⬇️
elasticluster/conf.py	71.42% <0%> (-1.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e664945...398d66d. Read the comment docs.