Add reading environment from Docker image

[catthehacker]

This change will add reading image definition and merging it's environment variables with our current env in style of:

• missing envvar, creates new one
• existing envvar is overwritten by our env
• PATH envvar is merged, container PATH is last to not overwrite any tools installed during workflow

It's not required for running our main container but it will be probably more intuitive if it will work like standard docker and this change is required for nested containers started in workflow

Fixes #757

[codecov]

Codecov Report

Merging #771 (93dc252) into master (0f04942) will increase coverage by 5.35%.
The diff coverage is 62.35%.

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
+ Coverage   49.27%   54.63%   +5.35%     
==========================================
  Files          23       23              
  Lines        2401     3725    +1324     
==========================================
+ Hits         1183     2035     +852     
- Misses       1090     1536     +446     
- Partials      128      154      +26     

Impacted Files	Coverage Δ
pkg/container/docker_build.go	0.00% <0.00%> (ø)
pkg/container/docker_run.go	6.67% <21.35%> (+4.75%)	⬆️
pkg/common/git.go	49.82% <31.81%> (-9.97%)	⬇️
pkg/model/planner.go	49.78% <42.50%> (+16.70%)	⬆️
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	79.48% <79.05%> (+10.53%)	⬆️
pkg/runner/run_context.go	81.85% <91.08%> (+5.44%)	⬆️
... and 25 more

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[joonas]

Glad to see this PR, I just recently came across an instance myself where the golang:1.17 container sets up a PATH variable, which unfortunately currently gets overridden in act unless an explicit PATH environment variable is set.

In turn, this results in the go not being available on the path for steps to use as it would be when running in GitHub Actions.

After spelunking a bit into the actions/runner codebase, I found that they are parsing it out from the docker inspect --format "{{range .Config.Env}}{{println .}}{{end}}" ... output.