Add proper support for working-directory & fix command builder

[catthehacker]

changelog:

• working-directory: is set through Docker now instead of adding cd %s to script
• workdir cases:
  • if starts /, it replaces workdir during that exec
  • if doesn't start with /, merge with current workdir
  • if empty, leave it alone and use what has been used before (container workdir)
• few formatting touches
• Print Docker executors to Info loglevel, when pulling (big) images this should let user know what is currently happening (currently it doesn't show anything about pull without verbose option)
• Replace pattern to script path after command has been re-split for Docker

Fixes #211
Fixes #748

[codecov]

Codecov Report

Merging #772 (e2557b0) into master (0f04942) will increase coverage by 4.58%.
The diff coverage is 62.29%.

@@            Coverage Diff             @@
##           master     #772      +/-   ##
==========================================
+ Coverage   49.27%   53.85%   +4.58%     
==========================================
  Files          23       23              
  Lines        2401     2670     +269     
==========================================
+ Hits         1183     1438     +255     
+ Misses       1090     1086       -4     
- Partials      128      146      +18     

Impacted Files	Coverage Δ
pkg/container/docker_build.go	0.00% <0.00%> (ø)
pkg/container/docker_run.go	1.77% <0.00%> (-0.16%)	⬇️
pkg/common/git.go	52.85% <30.15%> (-6.94%)	⬇️
pkg/runner/runner.go	68.96% <33.33%> (-7.51%)	⬇️
pkg/model/planner.go	51.85% <41.37%> (+18.76%)	⬆️
pkg/model/workflow.go	51.08% <62.68%> (+25.37%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/expression.go	85.38% <71.87%> (-1.26%)	⬇️
pkg/runner/step_context.go	78.78% <80.00%> (+9.83%)	⬆️
pkg/runner/run_context.go	80.36% <98.55%> (+3.95%)	⬆️
... and 7 more

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