chore(deps): bump alpine from 3.20.3 to 3.21.3 in /backend (#106)

Bumps alpine from 3.20.3 to 3.21.3.

---
updated-dependencies:
- dependency-name: alpine
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/Dockerfile

@@ -1,5 +1,5 @@
 
-FROM alpine:3.20.3 AS nodejs
+FROM alpine:3.21.3 AS nodejs
 
 # Install nodejs
 RUN addgroup -g 1000 node \