Bump alpine from 3.18.3 to 3.18.4 (#66)

Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/alpine:3.18.3
+FROM docker.io/alpine:3.18.4
 
 # https://git.alpinelinux.org/aports/log/community/postsrsd?h=3.18-stable
 ARG POSTSRSD_PACKAGE_VERSION=2.0.4-r1