README: add Gitlab job that uses a container with trivy

[TriplEight]

Hi, I was adding trivy instead of a broken built in GitLab for my use case clair, and made it work a simpler way than you describe it in README. This job takes ~15 sec given the image is cached.
Please correct me if I'm wrong somewhere.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #823 (54fdc43) into main (a00d719) will not change coverage.
The diff coverage is 95.83%.

@@           Coverage Diff           @@
##             main     #823   +/-   ##
=======================================
  Coverage   68.50%   68.50%           
=======================================
  Files          57       58    +1     
  Lines        2210     2210           
=======================================
  Hits         1514     1514           
  Misses        564      564           
  Partials      132      132           

Impacted Files	Coverage Δ
internal/app.go	98.38% <ø> (-0.10%)	⬇️
pkg/detector/ospkg/alpine/alpine.go	94.73% <ø> (ø)
pkg/github/github.go	58.00% <0.00%> (ø)
pkg/detector/library/python/advisory.go	100.00% <100.00%> (ø)
pkg/detector/library/python/compare.go	100.00% <100.00%> (ø)
pkg/detector/ospkg/redhat/redhat.go	93.54% <100.00%> (+0.21%)	⬆️
pkg/vulnerability/vulnerability.go	76.69% <100.00%> (+0.46%)	⬆️

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[knqyf263]

Thanks for your contribution. We already have a document regarding GitLab CI, but do you prefer using a container image?
https://github.com/aquasecurity/trivy#gitlab-ci

[TriplEight]

Thanks for your contribution. We already have a document regarding GitLab CI, but do you prefer using a container image?
https://github.com/aquasecurity/trivy#gitlab-ci

Yeah, that's why I've added it right under the existing one (can substitute if you find it better). My setup is faster, it doesn't use slow dind and doesn't download the binary on every run.

[TriplEight]

Are any more changes required here?

[knqyf263]

Sorry I was at a company-wide event last week. Thanks for your contribution!