Alpine: Upgrade to v3.21

This pull request upgrades to Alpine v3.21 for GCC 14. This contains additional performance improvements for rTorrent.

I would like to stay on PHP 8.3 for now, until PHP 8.4 receives more hotfixes. Furthermore, we need to make sure they are no ruTorrent issues with PHP 8.3 before upgrading to PHP 8.4.

Dockerfile

@@ -15,7 +15,7 @@ ARG DUMP_TORRENT_VERSION=302ac444a20442edb4aeabef65b264a85ab88ce9
 # v6.3-0.9.8-0.13.8
 ARG RTORRENT_STICKZ_VERSION=ddbbe8a03fb94cf95ab9a6d7fd5c60e51c0353ba
 
-ARG ALPINE_VERSION=3.20
+ARG ALPINE_VERSION=3.21
 ARG ALPINE_S6_VERSION=${ALPINE_VERSION}-2.2.0.3
 
 FROM --platform=${BUILDPLATFORM} alpine:${ALPINE_VERSION} AS src
@@ -205,9 +205,11 @@ RUN echo "net.core.rmem_max = 67108864" >> /etc/sysctl.conf \
   && sysctl -p
 
 # unrar package is not available since alpine 3.15
+# dhclient package is not available since alpine 3.21
 RUN echo "@314 http://dl-cdn.alpinelinux.org/alpine/v3.14/main" >> /etc/apk/repositories \
-  && apk --update --no-cache add unrar@314
-
+  && echo "@320 http://dl-cdn.alpinelinux.org/alpine/v3.20/main" >> /etc/apk/repositories \
+  && apk --update --no-cache add unrar@314 dhclient@320
+
 RUN apk --update --no-cache add \
     apache2-utils \
     bash \
@@ -216,7 +218,6 @@ RUN apk --update --no-cache add \
     brotli \
     ca-certificates \
     coreutils \
-    dhclient \
     ffmpeg \
     findutils \
     geoip \