Fix Docker runtime stage APK_MIRROR default for apk install.

ARG before FROM is not visible in RUN; set APK_MIRROR default inside the
runtime stage so sed does not produce broken https:///alpine URLs in CI.

Co-authored-by: Cursor <cursoragent@cursor.com>

Dockerfile

@@ -51,10 +51,9 @@ RUN --mount=type=cache,target=/go/pkg/mod \
     && go build -trimpath -ldflags="-w -s" -o /luminary-recover ./cmd/luminary-recover
 
 ARG IMAGE_PREFIX=
-ARG APK_MIRROR=mirrors.aliyun.com
 FROM ${IMAGE_PREFIX}alpine:3.20
 
-ARG APK_MIRROR
+ARG APK_MIRROR=mirrors.aliyun.com
 RUN sed -i "s/dl-cdn.alpinelinux.org/${APK_MIRROR}/g" /etc/apk/repositories \
     && apk add --no-cache ca-certificates tzdata tini
 

docker/Dockerfile.prebuilt

@@ -2,7 +2,7 @@
 ARG IMAGE_PREFIX=docker.1panel.live/library/
 ARG APK_MIRROR=mirrors.aliyun.com
 FROM ${IMAGE_PREFIX}alpine:3.20 AS runtime
-ARG APK_MIRROR
+ARG APK_MIRROR=mirrors.aliyun.com
 RUN sed -i "s/dl-cdn.alpinelinux.org/${APK_MIRROR}/g" /etc/apk/repositories \
     && apk add --no-cache ca-certificates tini