[Snyk] Security upgrade nginx from 1.21.1 to 1.23.3

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• backend-example/Dockerfile

We recommend upgrading to nginx:1.23.3, as this image has only 99 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	714	Exposure of Resource to Wrong Sphere SNYK-DEBIAN10-CURL-3065760	No Known Exploit
	714	Improper Authentication SNYK-DEBIAN10-CURL-3366770	No Known Exploit
	714	Directory Traversal SNYK-DEBIAN10-DPKG-2847944	No Known Exploit
	714	Out-of-bounds Read SNYK-DEBIAN10-LIBTASN16-3061094	No Known Exploit
	714	OS Command Injection SNYK-DEBIAN10-OPENSSL-2807585	No Known Exploit

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