Dockerfile: bump Alpine Linux version

[jimklimov]

Following https://stackoverflow.com/a/73374817/4715872 suggestions: Alpine signing key was rotated, so new package builds are not trusted by the old image

Closes: #622

NOTE: Whatever builds I do, it only runs "Testing in image" but not "Build runtime image" stage. So can't really tell if this fix works :\ Spun up Docker Desktop locally, but it is "loading metadata" for the past 6 minutes, so I don't know if it would proceed further.

UPDATE: Related to:

• PR self-tests fail in docker stage: mbuffer (no such package) #589
• Dockerfile: fail fast if mbuffer is not compatible/fetchable [#589] #635

[jimklimov]

Hm, on upstream/master builds it is still upset - now with perl version (we still reference a probably too-old one):

• https://github.com/oetiker/znapzend/actions/runs/7538237439/job/20518437972

#10 [runtime 2/4] RUN   apk add --no-cache zfs curl bash autoconf automake nano perl=5.30.3-r0 openssh &&   apk add --no-cache --repository http://dl-3.alpinelinux.org/alpine/edge/community/ mbuffer &&   ln -s /dev/stdout /var/log/syslog &&   ln -s /usr/bin/perl /usr/local/bin/perl
#10 0.794 fetch https://dl-cdn.alpinelinux.org/alpine/v3.19/main/x86_64/APKINDEX.tar.gz
#10 1.004 fetch https://dl-cdn.alpinelinux.org/alpine/v3.19/community/x86_64/APKINDEX.tar.gz
#10 1.458 ERROR: unable to select packages:
#10 1.462   perl-5.38.2-r0:
#10 1.462     breaks: world[perl=5.30.3-r0]
#10 1.462     satisfies: autoconf-2.71-r2[perl]
#10 1.462                automake-1.16.5-r2[perl]
#10 ERROR: process "/bin/sh -c apk add --no-cache zfs curl bash autoconf automake nano perl=${PERL_VERSION} openssh &&   apk add --no-cache --repository http://dl-3.alpinelinux.org/alpine/edge/community/ mbuffer &&   ln -s /dev/stdout /var/log/syslog &&   ln -s /usr/bin/perl /usr/local/bin/perl" did not complete successfully: exit code: 3
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But at least it seems 5.38.x is a suggested way to go - will post an updated PR :)