[docker] Bump nginx from 1.20-alpine to 1.31.2-alpine

Bumps nginx from 1.20-alpine to 1.31.2-alpine.

---
updated-dependencies:
- dependency-name: nginx
  dependency-version: 1.31.2-alpine
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Dockerfile

@@ -1,5 +1,5 @@
 # sample dockerfile for testing docker builds
-FROM nginx:1.20-alpine as base
+FROM nginx:1.31.2-alpine as base
 
 RUN apk add --no-cache curl