Security upgrade nginx from stable-alpine to mainline-alpine (#3563)

* fix: Dockerfile.ui to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE313-CURL-1533455
- https://snyk.io/vuln/SNYK-ALPINE313-CURL-1533456
- https://snyk.io/vuln/SNYK-ALPINE313-LIBGCRYPT-1315676
- https://snyk.io/vuln/SNYK-ALPINE313-LIBXML2-1300261
- https://snyk.io/vuln/SNYK-ALPINE313-LIBXML2-1300262

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

Dockerfile.ui

@@ -50,7 +50,7 @@ COPY cvat-canvas/ /tmp/cvat-canvas/
 COPY cvat-ui/ /tmp/cvat-ui/
 RUN npm run build
 
-FROM nginx:stable-alpine
+FROM nginx:mainline-alpine
 # Replace default.conf configuration to remove unnecessary rules
 RUN sed -i "s/}/application\/wasm wasm;\n}/g" /etc/nginx/mime.types
 COPY cvat-ui/react_nginx.conf /etc/nginx/conf.d/default.conf