fix: backend/test_certificate_signer/Dockerfile to reduce vulnerabili…

…ties

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE320-OPENSSL-7413532
- https://snyk.io/vuln/SNYK-ALPINE320-OPENSSL-7413532

backend/test_certificate_signer/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:lts-alpine
+FROM node:20.15.1-alpine3.20
 WORKDIR /app
 ENV PATH /app/node_modules/.bin:$PATH
 ENV CERTIFICATE_NAMESPACE "https://cowin.gov.in/credentials/testCertificate/v1"