fix: Dockerfile.ui to reduce vulnerabilities (cvat-ai#5286)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE316-CURL-3063712
- https://snyk.io/vuln/SNYK-ALPINE316-CURL-3063712
- https://snyk.io/vuln/SNYK-ALPINE316-CURL-3063713
- https://snyk.io/vuln/SNYK-ALPINE316-CURL-3063713
- https://snyk.io/vuln/SNYK-ALPINE316-LIBXML2-3050527

Dockerfile.ui

@@ -36,7 +36,7 @@ COPY cvat-canvas/ /tmp/cvat-canvas/
 COPY cvat-ui/ /tmp/cvat-ui/
 RUN yarn run build:cvat-ui
 
-FROM nginx:1.23.1-alpine
+FROM nginx:mainline-alpine
 # Replace default.conf configuration to remove unnecessary rules
 COPY cvat-ui/react_nginx.conf /etc/nginx/conf.d/default.conf
 COPY --from=cvat-ui /tmp/cvat-ui/dist /usr/share/nginx/html/