fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE314-LIBXML2-2421341

Author: snyk-bot

Dockerfile

@@ -1,4 +1,4 @@
-FROM nginx:1.21.4-alpine
+FROM nginx:1.21.5-alpine
 
 # CGit
 ARG CGIT_VERSION=1.2.3-r0