Set security context for trivy

[mlevesquedion]

This PR adds SecurityContexts to the trivy Job in order to run the job with the least amount of privilege possible, in partial fulfillment of #163.

I ran the job with and without the changes in a local kind cluster and I did not see a difference in the output.

To obtain the output, I ran:

kubectl create deployment nginx --image nginx:1.16
kubectl wait --for=condition=available deploy/nginx --timeout=60s
./bin/starboard scan vulnerabilityreports deployment/nginx
./bin/starboard get vulnerabilities deployment/nginx -o yaml >vulns.yaml

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Looking at issues on the trivy repo, I found this issue: aquasecurity/trivy#580, where running as a non-root user was causing trivy to be unable to run. It seems that this was due to using the Docker socket, which is not the case in starboard, so I don't think this could cause problems here.

[codecov]

Codecov Report

Merging #359 (02fd12a) into main (ab281e4) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
+ Coverage   66.35%   66.52%   +0.16%     
==========================================
  Files          58       58              
  Lines        3026     3041      +15     
==========================================
+ Hits         2008     2023      +15     
  Misses        769      769              
  Partials      249      249              

Impacted Files	Coverage Δ
pkg/trivy/plugin.go	74.47% <100.00%> (+1.41%)	⬆️

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[danielpacak]

Thank you @mlevesquedion for taking care of this one! LGTM