fix: rewrite how image env is merged

[catthehacker]

This is a replacement for #771 since i'm dumb the previous PR just adds environment to container and doesn't fix anything.

fixes #827
fixes #757 (for real this time)
fixes #161

[codecov]

Codecov Report

Merging #828 (efd1156) into master (0f04942) will increase coverage by 7.00%.
The diff coverage is 61.76%.

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
+ Coverage   49.27%   56.27%   +7.00%     
==========================================
  Files          23       23              
  Lines        2401     3730    +1329     
==========================================
+ Hits         1183     2099     +916     
- Misses       1090     1457     +367     
- Partials      128      174      +46     

Impacted Files	Coverage Δ
pkg/container/docker_run.go	5.63% <16.16%> (+3.70%)	⬆️
pkg/common/git.go	49.82% <31.81%> (-9.97%)	⬇️
pkg/model/planner.go	49.78% <42.50%> (+16.70%)	⬆️
pkg/container/docker_build.go	59.72% <50.00%> (+59.72%)	⬆️
pkg/model/workflow.go	52.14% <63.63%> (+26.42%)	⬆️
pkg/container/docker_pull.go	36.36% <64.00%> (+18.18%)	⬆️
pkg/runner/runner.go	74.46% <70.37%> (-2.01%)	⬇️
pkg/runner/expression.go	85.71% <72.09%> (-0.93%)	⬇️
pkg/runner/step_context.go	78.57% <76.71%> (+9.62%)	⬆️
pkg/runner/run_context.go	81.88% <91.17%> (+5.48%)	⬆️
... and 25 more

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[cplee]

@catthehacker is this testable?

[catthehacker]

definitely, will push test in few minutes

[catthehacker]

there is still massive TODO to cross-test testdata between all packages and some other work, but I will explain that in github team discussions because it's too much typing :)