Add docker context workaround to readme

[davetapley]

I already had to come look it up again once 😆

Per #583 (comment)

[mergify]

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[codecov]

Codecov Report

Merging #749 (3b56a13) into master (0f04942) will increase coverage by 1.17%.
The diff coverage is 56.42%.

@@            Coverage Diff             @@
##           master     #749      +/-   ##
==========================================
+ Coverage   49.27%   50.44%   +1.17%     
==========================================
  Files          23       23              
  Lines        2401     2593     +192     
==========================================
+ Hits         1183     1308     +125     
- Misses       1090     1145      +55     
- Partials      128      140      +12     

Impacted Files	Coverage Δ
pkg/container/docker_run.go	1.84% <0.00%> (-0.09%)	⬇️
pkg/model/workflow.go	25.29% <2.43%> (-0.43%)	⬇️
pkg/common/git.go	52.85% <30.15%> (-6.94%)	⬇️
pkg/runner/runner.go	68.96% <33.33%> (-7.51%)	⬇️
pkg/model/planner.go	34.56% <41.37%> (+1.48%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/step_context.go	74.70% <75.65%> (+5.75%)	⬆️
pkg/runner/run_context.go	79.84% <95.58%> (+3.44%)	⬆️
pkg/runner/command.go	90.58% <100.00%> (+2.35%)	⬆️
pkg/runner/expression.go	87.08% <100.00%> (+0.44%)	⬆️
... and 4 more

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