fix: return error instead of panic when walking in a nonexistent dir

[wolfogre]

fi fs.FileInfo will be nil when err error isn't.

[codecov]

Codecov Report

Merging #1667 (f8fc7ad) into master (4989f44) will increase coverage by 1.24%.
The diff coverage is 71.05%.

@@            Coverage Diff             @@
##           master    #1667      +/-   ##
==========================================
+ Coverage   61.22%   62.46%   +1.24%     
==========================================
  Files          46       48       +2     
  Lines        7141     7455     +314     
==========================================
+ Hits         4372     4657     +285     
- Misses       2462     2478      +16     
- Partials      307      320      +13     

Impacted Files	Coverage Δ
pkg/container/docker_cli.go	82.23% <ø> (ø)
pkg/container/docker_logger.go	52.08% <ø> (ø)
pkg/container/docker_pull.go	33.33% <ø> (ø)
pkg/container/docker_run.go	14.08% <0.00%> (+0.49%)	⬆️
pkg/container/docker_volume.go	0.00% <ø> (ø)
pkg/container/file_collector.go	37.30% <0.00%> (ø)
pkg/container/host_environment.go	0.00% <0.00%> (ø)
pkg/exprparser/functions.go	66.32% <0.00%> (-1.04%)	⬇️
pkg/model/workflow.go	42.11% <ø> (ø)
pkg/model/planner.go	44.71% <21.27%> (-4.12%)	⬇️
... and 30 more

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