Update the backup and restore image.

Use a newer version of Alpine.

backup/Dockerfile

-FROM docker.io/library/alpine:3.14
+FROM docker.io/library/alpine:3.21
 # hadolint ignore=DL3018
-RUN echo 'https://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories && \
-    echo 'https://dl-cdn.alpinelinux.org/alpine/edge/community' >> /etc/apk/repositories && \
+RUN echo '@testing https://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/repositories && \
+    echo '@community https://dl-cdn.alpinelinux.org/alpine/v3.21/community' >> /etc/apk/repositories && \
     apk add --update --no-cache \
         findutils \
-        skopeo \
-        reg \
+        skopeo@community \
+        reg@testing \
     ;
 COPY --chown=root:root backup /usr/local/bin/backup
 COPY --chown=root:root restore /usr/local/bin/restore