fix: examples/custom-builder/custom/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE316-ZLIB-2976176
- https://snyk.io/vuln/SNYK-ALPINE316-MUSL-8720632
- https://snyk.io/vuln/SNYK-ALPINE316-MUSL-8720632
- https://snyk.io/vuln/SNYK-ALPINE316-OPENSSL-3314624
- https://snyk.io/vuln/SNYK-ALPINE316-OPENSSL-3314641