fix: upgrade Alpine base packages in Docker images (#40252)

The `alpine:3.24` base image lags behind fixes published to the Alpine
repository.

Both images now run `apk upgrade` before `apk add`. Packages from `apk
add` are already current, so this only affects the preinstalled base
packages.

_Written by Claude (Opus 5.5)._

Dockerfile

@@ -52,7 +52,7 @@ FROM docker.io/library/alpine:3.24 AS gitea
 
 EXPOSE 22 3000
 
-RUN apk --no-cache add \
+RUN apk --no-cache upgrade && apk --no-cache add \
     bash \
     ca-certificates \
     curl \

Dockerfile.rootless

@@ -47,7 +47,7 @@ FROM docker.io/library/alpine:3.24 AS gitea-rootless
 
 EXPOSE 2222 3000
 
-RUN apk --no-cache add \
+RUN apk --no-cache upgrade && apk --no-cache add \
     bash \
     ca-certificates \
     dumb-init \