Extract run to a separate package

[tjamet]

Splitting the logic between extraction of current execution settings and actual runtime calls
This allows better testability as well as separation of concerns.
It also allows to have different implementations of the pure run interface allowing to support different runtimes whenever.

[codecov-io]

Codecov Report

Merging #98 into master will increase coverage by 6.02%.
The diff coverage is 81.7%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   40.16%   46.19%   +6.02%     
==========================================
  Files          13       15       +2     
  Lines         717      762      +45     
==========================================
+ Hits          288      352      +64     
+ Misses        405      383      -22     
- Partials       24       27       +3

Impacted Files	Coverage Δ
main.go	0% <0%> (ø)
cmd/run.go	78.68% <82.75%> (+43.16%)	⬆️
run/docker.go	84.31% <84.31%> (ø)

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