Allow setting ReadOnlyRootFilesystem

[senthilnathan]

Fixes #9980

[knative-prow-robot]

Hi @senthilnathan. Thanks for your PR.

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[codecov]

Codecov Report

Merging #10560 (7837454) into master (cf06222) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10560      +/-   ##
==========================================
+ Coverage   88.07%   88.10%   +0.02%     
==========================================
  Files         186      186              
  Lines        8830     8834       +4     
==========================================
+ Hits         7777     7783       +6     
+ Misses        811      810       -1     
+ Partials      242      241       -1     

Impacted Files	Coverage Δ
pkg/apis/serving/fieldmask.go	95.48% <100.00%> (ø)
pkg/apis/serving/metadata_validation.go	94.73% <100.00%> (-2.64%)	⬇️
pkg/apis/serving/v1/revision_validation.go	88.42% <100.00%> (+8.42%)	⬆️
pkg/apis/serving/v1/service_validation.go	100.00% <100.00%> (ø)
pkg/activator/net/revision_backends.go	90.41% <0.00%> (-0.92%)	⬇️
pkg/reconciler/configuration/configuration.go	88.28% <0.00%> (+1.56%)	⬆️
pkg/autoscaler/scaling/multiscaler.go	89.47% <0.00%> (+1.75%)	⬆️

---

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[markusthoemmes]

/ok-to-test
/lgtm

/assign @mattmoor @dprotaso

Not sure what our current stance about widening the contract is, but this seems quite benign to me.

[markusthoemmes]

Oh an thanks @senthilnathan 🎉

[mattmoor]

cc @evankanderson in case he has thoughts, but this does strike me as an extremely useful knob.

[evankanderson]

I'm not sure when this was added, but it seems like a good security addition

[mattmoor]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattmoor, senthilnathan

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[dprotaso]

/retest