docker: reduce image size by using multi-stage

[glimchb]

Signed-off-by: Boris Glimcher Boris.Glimcher@emc.com

[glimchb]

2x reduction:

$ docker images | head
REPOSITORY                                        TAG                 IMAGE ID            CREATED             SIZE
<none>                                            <none>              70141cce1f18        8 minutes ago       450MB
<none>                                            <none>              a5686720a676        8 minutes ago       811MB

[codecov-commenter]

Codecov Report

Merging #194 (88bf9da) into main (70a1fb2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #194   +/-   ##
=======================================
  Coverage   57.94%   57.94%           
=======================================
  Files          14       14           
  Lines        1612     1612           
=======================================
  Hits          934      934           
  Misses        678      678           

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