Adding support for proxy when preparing registry layers

[dnewhook]

Container will respect environment variables when connecting to a registry to prepare layers: HTTP_PROXY HTTPS_PROXY NO_PROXY This is based on https://github.com/quay/clair/pull/530

[codecov[bot]]

Codecov Report

Merging #22 (e4f7c6d) into master (3e9154f) will decrease coverage by 4.26%. The diff coverage is 25.33%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   50.35%   46.09%   -4.27%     
==========================================
  Files          13       14       +1     
  Lines         556      564       +8     
==========================================
- Hits          280      260      -20     
- Misses        259      286      +27     
- Partials       17       18       +1     

Impacted Files	Coverage Δ
pkg/etc/config.go	38.70% <ø> (-5.74%)	:arrow_down:
pkg/registry/client.go	0.00% <0.00%> (ø)
pkg/redisx/pool.go	25.67% <25.67%> (ø)
pkg/scanner/adapter.go	64.00% <0.00%> (-6.00%)	:arrow_down:
pkg/harbor/model.go	81.81% <0.00%> (-3.90%)	:arrow_down:
pkg/http/api/base_handler.go	72.22% <0.00%> (-1.70%)	:arrow_down:
pkg/http/api/v1/handler.go	92.12% <0.00%> (-0.47%)	:arrow_down:
pkg/job/job.go	100.00% <0.00%> (ø)
pkg/work/work.go	0.00% <0.00%> (ø)
... and 7 more

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