Bump alpine from 3.21.2 to 3.21.3 (#108)

Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/alpine:3.21.2
+FROM docker.io/alpine:3.21.3
 
 # https://dovecot.org/doc/NEWS
 # https://git.alpinelinux.org/aports/log/main/dovecot?h=3.21-stable