chore(deps): update alpine docker tag to v3.21

Author: renovate[bot]

Dockerfile

@@ -1,5 +1,5 @@
 ARG ARCH=
-FROM ${ARCH}alpine:3.20
+FROM ${ARCH}alpine:3.21
 
 LABEL Maintainer="Ernesto Serrano <[email protected]>" \
       Description="Lightweight container with Nginx & PHP-FPM based on Alpine Linux."

docker-compose.test.yml

@@ -3,7 +3,7 @@ services:
   app:
     build: .
   sut:
-    image: alpine:3.20
+    image: alpine:3.21
     depends_on:
       - app
     command: /tmp/run_tests.sh