Bump chainguard/nginx from `b70db73` to `91dba20` #1197

Job	Run time
backend-setup-gradle	5s
frontend-checks / code-quality-check-with-sonarqube	1m 40s
frontend-checks / build-frontend	26s
backend-build-image-and-scan / backend-build-image-and-scan	2m 23s
frontend-build-image-and-scan / frontend-build-image-and-scan	1m 17s
frontend-checks / e2e-test-frontend	5m 58s
frontend-checks / license-audit-frontend	19s
frontend-checks / unit-test-frontend	49s
frontend-checks / lint-and-format-check-frontend	17s
security-jobs / confirm-repository-secret-scanning-with-push-protection	3s
security-jobs / security-scan-with-trivy	18s
backend-checks / check-code-quality-with-sonarqube-backend	1m 43s
backend-checks / check-formatting-backend	30s
backend-checks / build-backend	40s
backend-checks / check-licenses	23s
backend-checks / test-backend	1m 15s
backend-push-image-to-registry	0s
frontend-push-image-to-registry	0s
frontend-push-reports-to-ris-reports	0s
deploy-staging	0s
backend-push-reports-to-ris-reports	0s
	18m 6s

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